The archaeal cell envelope.
ABSTRACT At first glance, archaea and bacteria look alike; however, the composition of the archaeal cell envelope is fundamentally different from the bacterial cell envelope. With just one exception, all archaea characterized to date have only a single membrane and most are covered by a paracrystalline protein layer. This Review discusses our current knowledge of the composition of the archaeal cell surface. We describe the wide range of cell wall polymers, O- and N-glycosylated extracellular proteins and other cell surface structures that archaea use to interact with their environment.
- SourceAvailable from: Samar Damiati[Show abstract] [Hide abstract]
ABSTRACT: The most important aspect of synthetic lipid membrane architectures is their ability to study functional membrane-active peptides and membrane proteins in an environment close to nature. Here, we report on the generation and performance of a biomimetic platform, the S-layer supported lipid membrane (SsLM), to investigate the structural and electrical characteristics of the membrane-active peptide gramicidin and the transmembrane protein α-hemolysin in real-time using a quartz crystal microbalance with dissipation monitoring in combination with electrochemical impedance spectroscopy. A shift in membrane resistance is caused by the interaction of α-hemolysin and gramicidin with SsLMs, even if only an attachment onto, or functional channels through the lipid membrane, respectively, are formed. Moreover, the obtained results did not indicate the formation of functional α-hemolysin pores, but evidence for functional incorporation of gramicidin into this biomimetic architecture is provided.International Journal of Molecular Sciences 01/2015; 16(2):2824-38. · 2.46 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: The common idea of typical cell wall architecture in archaea consists of a pseudo-crystalline proteinaceous surface layer (S-layer), situated upon the cytoplasmic membrane. This is true for the majority of described archaea, hitherto. Within the crenarchaea, the S-layer often represents the only cell wall component, but there are various exceptions from this wall architecture. Beside (glycosylated) S-layers in (hyper)thermophilic cren- and euryarchaea as well as halophilic archaea, one can find a great variety of other cell wall structures like proteoglycan-like S-layers (Halobacteria), glutaminylglycan (Natronococci), methanochondroitin (Methanosarcina) or double layered cell walls with pseudomurein (Methanothermus and Methanopyrus). The presence of an outermost cellular membrane in the crenarchaeal species Ignicoccus hospitalis already gave indications for an outer membrane similar to Gram-negative bacteria. Although there is just limited data concerning their biochemistry and ultrastructure, recent studies on the euryarchaeal methanogen Methanomassiliicoccus luminyensis, cells of the ARMAN group, and the SM1 euryarchaeon delivered further examples for this exceptional cell envelope type consisting of two membranes.Frontiers in Microbiology 11/2014; 5:624. · 3.94 Impact Factor
- Colloids and surfaces B: Biointerfaces 02/2015; 126:303-312. · 4.29 Impact Factor
Although originally proposed by Woese1 in 1977, it
was some time before most microbiologists accepted
the premise that archaeal microorganisms might be
strikingly different from bacterial microorganisms.
Eventually, it was considered to be justified to designate
Archaea as a distinct domain that is separate and unique
from Bacteria and Eukarya in the tree of life2. Studies
of cell surface structures of early archaeal isolates (for
example, of halobacteria and methanogens) revealed
some unusual features that are not present in bacteria,
such as a lack of peptidoglycan (also known as murein)
and the presence of a crystalline protein layer on the cell
surface3,4. However, these features were viewed as curi-
osities, and their taxonomic implications were not fully
realized until after Woese developed his new concept
of the Archaea being a novel domain. More-detailed
studies of the archaeal cell envelope during the 1970s
and 1980s resulted in the recognition of a number of
archaea-specific features, including a different lipid
composition to bacteria (Box 1) and the lack of a gen-
eral cell wall polymer, resulting in insensitivity to the
most common bacterial cell wall targeting antibiotics.
Identification of these profound differences made the
gap between Archaea and Bacteria irrevocably clear.
Although archaea were initially cultivated from a
range of extreme environments, including acidic and
alkaline hot springs, salt brines and deep sea black smokers,
they have now been detected in virtually all of the niches
that are inhabited by bacteria5. Therefore, understand-
ing the archaeal cell envelope is of utmost importance
in order to understand how archaea interact with their
natural environments, and how this enables them to
thrive in a wide range of habitats.
This Review discusses our current understanding
of the composition of the archaeal cell envelope, includ-
ing the different envelope polymers and surface layer
proteins. In addition, we discuss cell surface structures
such as flagella and pili that link the cell surface to abiotic
and biotic surfaces in the environment of archaea.
Cell surface proteins
Although only a subgroup of archaea possess cell enve-
lope polymers, most possess a proteinaceous layer that
surrounds the cell and is proposed to contribute to cell
shape and osmoprotection; however, some archaeal
species do survive without these structures (FIG. 1).
S-layer. Surface layer (S-layer) proteins, which make up
the S-layer cell wall structure, are widespread and can be
found in almost all archaea as well as in a range of spe-
cies from all major phylogenetic groups of the Bacteria6,7.
Indeed, it seems that only some members of the gen-
era Thermoplasma, Halococcus, Methanobrevibacter,
Natronococcus, Methanosphaera, Ignicoccus and the
species Thermosphaera aggregans lack an S-layer (FIG. 2;
Supplementary information S1 (table). Because of its
widespread distribution and simple composition, the
S-layer might be the earliest cell wall structure to have
evolved. In some archaea (for example, Sulfolobus spp.)
S-layer proteins are the sole cell envelope constituent,
whereas in other archaea the cell envelope consists
of multiple polymers, including the polysaccharides
pseudomurein and methanochondroitin, and can also
contain additional S-layer proteins. The first archaeal
S-layer protein was discovered in 1956 on examina-
tion of Halobacterium salinarum cells using electron
Archaeal S-layers are mostly composed of a single
protein or glycoprotein species with an apparent relative
molecular mass of 40–200 kDa, which in many cases is
associated with the cytoplasmic membrane. In haloarchaea,
methanogens, Staphylothermus spp. and Thermoproteus
spp. the main protein constituent of the S-layer is anchored
by its carboxy-terminal transmembrane domain to the
Molecular Biology of Archaea,
Max Planck Institute for
D-35043 Marburg, Germany.
Correspondence to S.-V.A.
A type of hydrothermal vent,
which appears as a black
chimney-like structure that
emits a cloud of black material
composed of high levels of
sulphur-bearing minerals, or
The archaeal cell envelope
Sonja-Verena Albers and Benjamin H. Meyer
Abstract | At first glance, archaea and bacteria look alike; however, the composition of the
archaeal cell envelope is fundamentally different from the bacterial cell envelope. With just
one exception, all archaea characterized to date have only a single membrane and most are
covered by a paracrystalline protein layer. This Review discusses our current knowledge of
the composition of the archaeal cell surface. We describe the wide range of cell wall
polymers, O- and N-glycosylated extracellular proteins and other cell surface structures
that archaea use to interact with their environment.
414 | junE 2011 | VoluME 9
A form of symmetry displayed
by S-layer proteins, in which
the proteins do not lie at a
right angle, or multiples of
a right angle, to each other.
A form of symmetry displayed
by S-layer proteins, in which
the proteins lie at a right angle,
or multiples of a right angle, to
cytoplasmic membrane (FIG. 3a). The transmembrane
domain is often preceded by a stretch of serine/threonine
residues that are often glycosylated. In some cases, the
S-layer is composed of two S-layer proteins; for example, in
Sulfolobales spp. the S-layer is composed of the large outer
protein SlaA and the small membrane-bound protein
A remarkable feature of S-layer proteins is their
intrinsic ability to assemble into a two-dimensional crys-
talline protein or glycoprotein array, which makes their
potential use for nanotechnology applications an attrac-
tive prospect7. The two-dimensional crystalline S-layer
can have oblique symmetry (p1 or p2), square symmetry
(p4) or hexagonal symmetry (p3 or p6) (FIG. 3b). Hexagonal
symmetry is most predominant in archaeal S-layers (see
Supplementary information S1 (table)). Depending on
the S-layer structure, a latticed repeating unit can be
composed of up to six identical S-layer proteins, which
contain two or more reoccurring pore geometries.
These pores are 2–8 nm in diameter and can occupy up
to 70% of the cell envelope. The centre–centre spacing
of S-layer units ranges from ~10 nm (Methanococcus
voltae) to 36 nm (Staphylothermus marinus). Most
S-layers are 5–25 nm thick and are fairly smooth on their
outer surface with a more corrugated inner surface. A
summary of most characterized archaeal S-layers is given
in Supplementary information S1 (table).
Proteinaceous sheaths. Methanospirillum hungatei (FIG. 2)
and Methanosaeta concilii probably have the most com-
plex cell envelope of any archaea that have been described
to date. The rod-shaped cells of these methano gens exist
as filamentous chains that are enclosed by a unique tubu-
lar paracrystalline proteinaceous sheath11,12. This sheath
differs from the normal S-layer proteins in that it exhibits
a very low porosity13 and has strong recurrent covalent
links containing cysteine, which results in an unusually
stable layer that resists dissociation by conventional
treatments11,14–16. Moreover, the sheath encloses the linear
cell chain community and not each individual cell.
Although the protein sheaths in these two species
display general morphological similarity (that is, both
species seem to have hoop-like structures and are
arranged in the same symmetry11,14), chemical analyses
of isolated M. concilii sheaths revealed a lower ratio of
acidic and basic amino acids than in M. hungatei sheaths.
Additionally, the two methanogens differ in the mor-
phology of their so-called spacer plugs, which separate
individual cells from each other. Whereas M. concilii
spacer plug subunits form concentric rings11,17, M. hun
gatei spacer plug subunits are arranged in a multilayered,
p6 symmetry11,18. Furthermore, cells of each organism
are surrounded by an inner cell wall, which is difficult
to characterize because of its instability, especially after it
is isolated from the sheath. In M. hungatei, the inner cell
wall is composed of S-layer proteins that are arranged as
crystals with a hexagonal symmetry. In M. concilii each
cell is surrounded by an amorphous granular layer that
is presumed to be analogous to the S-layer.
Cell envelope polymers
Gram-staining is the traditional tool used to classify bacte-
ria, which can be divided into two distinct groups: Gram-
positive and Gram -negative19. In general, Gram-negative
bacteria (FIG. 2) possess an outer asymmetric bilayered
membrane composed of two leaflets (an outer one con-
taining lipopolysaccharide(s) and an inner one mainly
containing phospholipids), a gel-like periplasm containing
peptidoglycan and a cytoplasmic membrane composed of
two phospholipid leaflets. Gram-positive bacteria have
a thick, amorphous, multilayered coat of peptidoglycan,
teichoic acid and lipoteichoic acid as their cell wall (FIG. 2)
and in some cases harbour S-layer glycoproteins at the
Bacterial lipidsArchaeal lipids
Box 1 | Lipids from archaea and bacteria
The lipids found in the archaeal membrane are fundamentally different from those
found in eukaryotic and bacterial membranes. In eukaryotes and bacteria, the glycerol
moiety is ester-linked to an sn-glycerol-3-phosphate backbone, whereas in archaea the
isoprenoid side chains are ether-linked to an sn-glycerol-1-phosphate moiety. The sn1
stereochemistry of the glycerol backbone is a truly archaeal feature, as ether lipids occur
in minor amounts in eukaryotes and bacteria. The common bilayer-forming lipids in
bacteria are phophatidylglycerol (upper lipid) and phosphatidylethanolamine (lower
lipid) (see the figure, part a). Part b of the figure shows the structure of monolayer-forming
tetraether lipids; for example, the glycophospholipid from the thermoacidophilic
archaeon Thermoplasma acidophilum, in which the hydrophobic core consists of C40C40
caldarchaeol. Part c of the figure shows a bilayer formed of archaeal diether lipids, which
can be found, for example, in Halobacteriales. The hydrophobic core consists of C20C20
archaeol isoprenoids. The headgroups of phospholipids can be a range of polar
compounds — for example, glycerol, serine, inosine, ethanolamine, myo-inositol or
aminopentanetetrols. Glycolipids also exhibit a range of sugar residues — for example,
glucose, mannose, galactose, gulose, N-acetylglucosamine or combinations thereof.
For details, see ReF.141.
nATuRE REVIEWS | MicrobioLogy
VoluME 9 | junE 2011 | 415
Thermoproteales Sulfolobales Desulfococcales Thermococcales Methanococcales Thermoplasmatales Halobacteriales Methanosarcinales Methanomicrobiales
Nitrosopumilus maritimus SCM1
Thermoproteus tenax strain YS44
Pyrobaculum islandicum DSM 4184
Candidatus Korarchaeum cryptofilum
Thermoproteus neutrophilus V24Sta
Metallosphaera prunae Ron12/II
Metallosphaera sedula DSM 5348
Sulfolobus acidocaldarius DSM 639
Sulfolobus tokodaii str. 7
Sulfolobus solfataricus P2
Ignicoccus hospitalis KIN4/I
Staphylothermus marinus F1
Aeropyrum pernix K1
Pyrolobus fumarii 1A
Hyperthermus butylicus DSM 5456
Nanoarchaeum equitans Kin4−M
Methanococcus voltae A3
Methanococcus vannielii SB
Methanococcus maripaludis C7
Methanobrevibacter ruminantium M1
Methanogenium tationis DSM 2702
Methanogenium marisnigri DSM 1498
Methanogenium cariaci DSM 1497
Methanoculleus marisnigri JR1
Methanospirillum hungatei JF−1
Methanolobus tindarius DSM2278
Methanosarcina acetivorans C2A
Methanosarcina mazei Go1
Methanosaeta concilii Opfikon
Natronococcus occultus SP4
Halobacterium salinarum R1
Haloferax volcanii DS2
Haloquadratum walsbyi C23
Haloquadratum walsbyi HBSQ001
Haloarcula japonica TR−1
Picrophilus torridus DSM 9790
Archaeoglobus veneficus DSM11195
Archaeoglobus fulgidus DSM 4304
Ferroglobus placidus DSM 10642
Pyrococcus furiosus DSM 3638
Methanopyrus kandleri AV19
Nature Reviews | Microbiology
Not analysed so far
A form of symmetry displayed
by S-layer proteins, in which
the proteins are at an angle of
60° or 120° to each other.
A lattice structure that is highly
ordered over short distances
but lacks long-range ordering
at least in one direction.
All amino acids, except glycine,
can exist as either one of two
optical isomers, which are
mirror images of each other.
These forms are called l- or
d-amino acids. only l-amino
acids can be recognized in the
translation process to be used
for the synthesis of proteins.
d-amino acids are more rare
and can be found, for example,
in bacterial peptidoglycan.
Microorganisms that thrive in
alkaline environments and
require a pH higher than 9 for
outermost boundary of the peptido glycan layer; for
example, in the Bacillus stearothermophilus S-layer20,21.
only few archaea possess pseudomurein, a poly-
mer with a thickness of ~15–20 nm that is similar to
bacterial peptidoglycan22 (FIGS 1,2). However, unlike
the N-acetylmuramic acid with a β-1,4 linkage to d-N-
acetylglucosamine (GlcnAc) that is found in bacterial
peptidoglycan, the pseudomurein oligosaccharide back-
bone is composed of l -N-acetyltalosaminuronic acid with
a β-1,3 linkage to GlcnAc. Additionally, the amino acid
interbridge lacks d-amino acids. Instead, the interbridge is
frequently composed of three l-amino acids (glutamic
acid, alanine and lysine). Interestingly, although a bio-
synthetic pathway for pseudomurein in archaea has been
proposed23, no proteins from pseudomurein-producing
archaea share homology with the bacterial proteins that
are involved in peptidoglycan biosynthesis and assembly24,
suggesting that the two pathways evolved separately25.
The cell wall polymer methanochondroitin is pro-
duced by aggregated cells of Methanosarcina spp.26 but
not by single Methanosarcina spp. cells, which only have
the protein S-layer that is adjacent to the cell membrane27
(FIG. 2). Methanochondroitin is a fibrillar polymer com-
posed of a trimer repeat of two N-acetylgalactosamines
(GalnAc) and one glucuronic acid (GlcA). It is simi-
lar to chondroitin, which is produced by vertebrates as
a major component of the connective tissue matrix28.
However, methanochondroitin differs from chondroi-
tin in the molar ratio of GalnAc to GlcA (2/1 instead of
1/1) and in that it is not sulphated. Methanochondroitin
is synthesized from nucleotide-activated uDP-GalnAc
and uDP-GlcA precursors, which are assembled into a
repeating uDP-GalnAc–GalnAc–GlcA unit29 and trans-
ferred onto the lipid carrier undecaprenyl, from which the
glycan chain is assembled by repeated addition.
The cell envelope of the highly alkaliphilic and halo-
philic species Natronococcus occultus (which lives under
conditions of 3.5 M salt and pH 9.5–10)30 consists of a
glutaminylglycan polymer. This polymer has a back-
bone composed of a poly-γ-glutamine chain with a
length of ~60 monomers that is covalently linked to
two different oligosaccharides at a molar ratio of 1/1.
The first oligosaccharide is a GlcnAc pentamer with
an α-1,3 linkage at the reducing end to a chain of more
than five galacturonic (GalA) acid monomers that are
joined on with β-1,4 linkages. The other oligosaccha-
ride is a β-1,3 GalnAc dimer that is α-1,4 linked at its
non-reducing end to a glucose (Glc) dimer31.
Figure 1 | Diversity of surface envelope types across the domain of Archaea. The phylogenetic tree is based on
the alignment of the full length 16S RNA sequence, in agreement with recent analyses142,143. Evolutionary distances
are not given. S-layer, surface-layer.
416 | junE 2011 | VoluME 9
200 nm 200 nm 500 nm
Gram-positive cell wallGram-negative cell wall
Ihomp124 nm pore
Nature Reviews | Microbiolog
Microorganisms that require
high concentrations of salt for
Figure 2 | cell wall profiles of different archaea. a,b | Electron micrographs of ultra-thin sections of the euryarchaeote
Methanocaldococcus villosus (a) and the crenarchaeote Metallosphaera prunae (b). c,d | Electron micrographs of a
freeze-etched cell (c) and a thin-section cell (d) of Ignicoccus hospitalis144. e | Schematic side view of cell wall profiles from
different archaea. Pseudoperiplasmic space is shown in blue. f | Schematic of bacterial cell walls. Gram-positive bacteria
have a thick, amorphous, multilayered coat of peptidoglycan, teichonic and lipoteichonic acid as their cell wall and in
some cases have surface-layer (S-layer) glycoproteins as the outermost layer above the peptidoglycan (also known as
murein), for example, in Bacillus stearothermophilus20,21. Gram-negative bacteria have an outer asymmetric bilayer
membrane composed of two leaflets, an outer one containing lipopolysaccharides (LPSs), and an inner one containing
mainly phospholipids, a gel-like periplasm containing peptidoglycan and the cytoplasmic membrane. CM, cytoplasmic
membrane; SL, S-layer. Images in parts a–d courtesy of R. Rachel, University of Regensburg, Germany.
nATuRE REVIEWS | MicrobioLogy
VoluME 9 | junE 2011 | 417
Nature Reviews | Microbiology
18 nm20.5 nm
Hole 5 nm
Another extremely halophilic species, Halococcus
morrhuae, has a rigid cell envelope, which is a homog-
enous layer with a thickness of 50–60 nm32. It is com-
posed of a highly sulphated heteropolysaccharide33,34, of
which the principle components are GalA, Glc, GlcA,
mannose (Man), galactose, glucosamine, galactosamine
and glucosaminuronic acid. The polymer also contains
N-acetylated amino sugars and high levels of sulphated
subunits35. The heteropolysaccharide also contains
substantial amounts of glycine, which is thought to
build glycyl bridges between the amino groups of glu-
cosamines and the carboxyl groups of uronic acid resi-
dues33. Although a model for the structure of this cell
surface matrix has been suggested (see Supplementary
information S2 (table))34, its biosynthesis has yet to be
The extremely halophilic Haloquadratum walsbyi
possesses an unusual ultra-thin square cell morphology
(approximately 2 × 2 × 0.2 μm)36. Its ability to tolerate
conditions of very low water activity (near the known
limits at which life can exist) in saltern crystallizer ponds
may rely as much on its unique cell shape as on its (pro-
tective) cell envelope. H. walsbyi has a typical S-layer cell
envelope over its cytoplasmic membrane. Additionally,
because homologues of genes coding for the bacterial
biosynthesis protein complex CapBCA (which produces
poly-γ-glutamate)37 have been identified in the H. walsbyi
genome36, it is likely that H. walsbyi is surrounded by a
poly-γ-glutamate capsule. However, its ability to cope
with conditions of up to 2 M MgCl2 probably relies on
the presence of an additional surface protein called halo-
mucin, which is a 9,159 amino acid protein (the larg-
est archaeal protein known to date). It has a remarkable
convergence in terms of amino acid composition and
domain organization36 with mammalian mucin, which
acts as a shield against dehydration of various tissues,
such as the bronchial epithelium and the eyes38.
Mostly because of the lack of genetic tools in the past,
the specific function is not known for most of the dif-
ferent archaeal cell envelope polymers. nonetheless, in
at least the strains in which these polymers are the sole
component of the cell envelope, it can be speculated
that they are involved in maintaining cell morphology,
osmoprotection and mechanical stability.
Archaea with different cell envelopes. Interestingly,
members of the Thermoplasmatales, such as Ferroplasma
acidophilum39 and other Thermoplasma spp., do not pos-
sess a cell wall. Thermoplasma spp. live under extremely
low pH values (pH 1–2) and at temperatures of around
60 °C40. Despite this harsh environment, Thermoplasma
spp. lack a distinct cell wall, resulting in a highly pleo-
morphic shape41. Therefore, it is likely that the mainte-
nance of cellular integrity relies on membrane-associated
glycoproteins42 and lipoglycans43,44. The abundance of
sugar residues (mainly Man) that are attached to mem-
brane proteins (see Supplementary information S2
(table)) and lipids is thought to constitute the protective
Figure 3 | Models of the archaeal S-layer. a | Side view of the assembly of surface-layer (S-layer) proteins in different
archaea (Halobacteria145, Sulfolobales145, Thermoproteus spp.145 and Staphylothermus marinus146; for details see also
Supplementary information S1 (table)). b | Top view of the lattice structure of different S-layers: S-layer proteins are shown
in grey, and pores or holes in white. The red perimeter shows a single repeating unit in the cases of the p3, p4 and p6
symmetry. Sulfolobales display p3 symmetry147, Desulphurococcus mobilis displays p4 symmetry148 and Thermoproteus
tenax displays p6 symmetry149,150. N, N-linked glycosylation; O; O-linked glycosylation.
418 | junE 2011 | VoluME 9
A cell-surface coat made of
glycoproteins and glycolipids.
slime coat called the glycocalyx, which may fulfil some
of the functions of a conventional cell wall.
Ignicoccus spp. represent another interesting excep-
tion in the archaeal domain in that they are the only spe-
cies that possess two membranes and no S-layer45 (FIG. 2).
The outer membrane is an asymmetric bilayer mem-
brane and contains a 6 kDa protein, Ihomp1, that forms
large stable complexes46, thereby imitating both a porin
and an S-layer. The distance between the outer and inner
membranes can vary between 20–1,000 nm, and this
periplasmic space contains a multitude of vesicles that
bud from the inner membrane and fuse with the outer
membrane. Immunolocalization studies have revealed
that the two enzymes that are vital for energy production
and conservation, H2:sulphur oxidoreductase and A1Ao
ATP synthase, are present in the outer membrane. This is
a good indication that ATP is produced primarily in the
periplasmic space and not in the cytoplasm47, meaning
that efficient ATP and ADP carriers must be present in
the inner membrane to ensure that intracellular ATP is
present in the cytoplasm. It will be interesting to deter-
mine how metabolic enzymes are translocated across the
inner membrane into the periplasm from the cytosol,
and whether the outer membrane proteins are shuttled
to the outer membrane by the vesicles or by another
Some archaeal species (for example, some Thermococcus
spp.48, some Sulfolobus spp.49 and Aciduliprofundum
boonei50) have been reported to release membrane vesi-
cles into the medium. In general, the function of these
secreted vesicles is unknown; however, it has been shown
that these vesicles were associated with antimicrobial
activity in Sulfolobales. Moreover, the protein composi-
tion of Sulfolobales vesicles is markedly different from that
of the cytoplasmic membrane, and there is an accumula-
tion of endosomal sorting complex required for transport
III (ESCRT-III) proteins, suggesting that vesicle formation
is an active process49. This is similar to some members of
the Bacteria and Eukarya, which can use vesicles released
from cells for the transfer of signals between cells.
Glycosylation of extracellular proteins
All of the surface-exposed archaeal proteins that have
been studied, including the S-layer proteins51–58, flag-
ellins59, pilins60 and sugar binding proteins61, are post-
translationally modified. The S-layer proteins in different
haloarchaea are lipid-modified at serine/threonine resi-
dues, probably to ensure membrane anchoring62,63.
Most S-layer proteins are either N- or O-glycosylated,
but both modifications can also be present on one pro-
tein at the same time (Supplementary information S1,
N-glycosylation. In the past few years, substantial
progress has been made in describing the archaeal
N-glycosylation pathway of the euryarchaeota M. vol
tae52,64, Methanococcus maripaludis64–66 and Haloferax
volcanii67–69. Although Eukarya, Bacteria and Archaea
all seem to share certain features in their N-glycosylation
pathways, the archaeal pathway is a mosaic of the
eukaryal and bacterial systems (FIG. 4). A clear example
of this is the glycosylation consensus n-x-S/T (n-x-T/
S/n/l/V in H. salinarum51 (where x ≠ P)) that Eukarya
and Archaea share, which is dissimilar to the extended
bacterial motif (D/E-z-n-x-S/T (where x or z ≠ P)) for
the β-glycosylamide linkage of the oligosaccharide to
The archaeal N-glycosylation pathway starts at the
inner side of the cytoplasmic membrane, where specific
glycosyltransferases facilitate a step-wise assembly of
oligosaccharide chains on lipid carriers using nucleotide-
activated sugars precursors. After the attachment of the
final sugar to the chain, the whole oligosaccharide along
with the lipid carrier is flipped across the membrane.
The whole oligosaccharide is then transferred en bloc
from the lipid carrier onto the nascent protein by the
oligosaccharyltransferase AglB (FIG. 4). In Archaea and
Bacteria, only a single gene product (AglB and PglB,
respectively) is needed for the oligosaccharyltrans-
ferase reaction71,72, whereas in Eukarya, the oligosac-
charyltransferase (oST)-complex is composed of nine
membrane-bound protein subunits, including the STT3
Most archaeal N-linked glycans are composed of 3–5
sugar residues and are mostly assembled unbranched,
and are therefore relatively simple (see Supplementary
information S2 (table)). By contrast, bacterial or eukary-
otic oligosaccharides are made of more than seven
sugar residues and are always branched. Eukaryotic
N-linked oligosaccharides are composed of a highly
conserved 14-membered branched glycan-tree —
(GlcnAc)2Man(Man(Man2)(Man2))(Man3Glc3) — which
can contain complex modifications, whereas bacterial
oligosaccharides (for example, from Campylobacter
jejunii) are composed of seven sugar residues that are
branched — (GalnAc)2(Glc)(GalnAc)3Bac (in which
Bac is bacilosamine (2,4-diamino-2,4,6-tri deoxy-
glucopyranose)). Recently, it was shown that the oli-
gosaccharide of the S-layer of the thermo acidophilic
crenarchaeote Sulfolobus acidocaldarius consists of a
branched, six-membered glycan tree, a more complex
structure than that found in many non-thermophilic
archaea58. Furthermore, the thermophilic archaeon
Pyrococcus furiosus was also able to transfer branched
oligosaccharides to N-glycosylation sites through the
activity of AglB76. Whether thermophilic archaea con-
tain more branched glycans on their surfaces than non-
thermophilic archaea in order to better protect and
stabilize their extracellular proteins has yet to be
proven. Indeed, at least one thermophilic archaeon,
Methanothermus fervidus, possesses only unbranched
glycan trees composed of four sugar residues77–79.
S-layer proteins from the hyperthermophiles
Methanocaldococcus jannaschii and Methanotorris igneus
have a higher number of potential N-glycosylation sites
compared with mesophilic species, suggesting a role for
glycosylation in thermoadaptation. The C terminus of the
S-layer protein from the thermophile S. acidocaldarius
contains 11 of 33 potential N-glycosylation sites; of these,
9 have been shown to be glycosylated58. However, ther-
mal stabilization of S-layer proteins may not depend
merely on the level of glycosylation and the composition
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VoluME 9 | junE 2011 | 419
Eukarya (e.g. Saccharomyces cerevisiae)
Bacteria (e.g. Campylobacter jejuni)
220 or 262 Da sugar
Methylester of hexuronic acid
A mechanism to enable the
growth of organisms at high
adaptation of, for example,
proteins, lipids and other
of the N-linked glycans, but could also be attributed
to phosphorylation, salt-bridging or covalent
A comparison of S-layer proteins from the moder-
ate halophile H. volcanii with those from the extreme
halophilic H. salinarum showed that there is 40.5%
identity in amino acid composition between the two
species, with H. salinarum having only a slightly higher
number of acidic residues. However, the N-glycan com-
position of H. volcanii differs from that of H. salinarum
Figure 4 | Schematic model of the N-glycosylation pathway in the three domains of life. Major differences between
the N-glycosylation pathways in the three domains of life are depicted. a | In eukaryotes (exemplified by Saccharomyces
cerevisiae) the N-glycosylation pathway starts at the membrane of the endoplasmic reticulum (ER), with the assembly by
specific glycosyltransferases of nucleotide-activated sugar precursors onto the lipid carrier dolichol. In the eukaryotic
system the enzymes involved are the ALG enzymes. The preliminary oligosaccharide is flipped across the ER membrane
by an unidentified flippase in an ATP-independent manner. The oligosaccharide is further enlarged by sugars from
dolichol-phosphate activated precursors, thus creating a 14-membered branched oligosaccharide, which is conserved in all
eukaryotes. The oligosaccharide is transferred en bloc by the oligosaccharyltransferase (OST) complex (composed of nine
subunits: Ost6, Ost3, Ost4, Stt3, Ost2. Wbp1, Swp1, Ost5 and Ost1) to the asparagine residue in the specific glycosylation
motif N-x-S/T (where x ≠P) of a nascent protein. The oligosaccharide can be further modified; for example, it can be
trimmed in the ER or elaborated in the Golgi apparatus. For details see ReFS 151,152. b | In archaea the N-glycosylation
pathway starts at the cytoplasmic side of the plasma membrane. Currently, it is not clear whether only nucleotide-activated
or also dolichol-phosphate activated sugar precursors contribute to the assembly of the oligosaccharide. The activated
sugars are successively added by specific glycosyltransferases onto the lipid carrier dolichol. In contrast to the eukaryotic
system, the oligosaccharide sequence composition in archaea is not conserved. The oligosaccharide is flipped across the
cytoplasmic membrane by an unknown flippase. The oligosaccharide is transferred en bloc by a single OST protein, AglB, (to
the asparagine residue in the specific glycosylation motif N-x-S/T (where x ≠ P) of a nascent protein. The N-glycosylation
pathway of Methanococcus maripaludis153 is not depicted here. The * marks a point in the pathway at which in some strains
an additional 220 or 262 Da sugar is attached to the oligosaccharide64. For details, see ReF.154. c | In bacteria, exemplified by
Campylobacter jejuni, the N-glycosylation pathway starts on the cytoplasmic side of the plasma membrane, with the assembly
of nucleotide-activated sugar precursors onto the lipid carrier undecaprenyl through specific glycosyltransferases. The sugar
attaching the oligosaccharide to the lipid carrier is bacillosamine (Bac; 2,4-diamino-2,4,6-trideoxygluco pyranose)155.
The assembled oligosaccharide is flipped across the membrane by flippase PlgK in an ATP-dependent manner. The
oligosaccharide is transferred en bloc by a single oligosaccharyltransferase protein, PglB, onto an asparagine residue
in an extended glycosylation motif D/E-x-N-z-S/T (where x and z ≠ P). For details see ReFS 85,156. GalNAc,
N-acetylgalactosamine; GlcNAc, d-N-acetylglucosamine; Glc, glucose; Man, mannose.
420 | junE 2011 | VoluME 9
Microorganisms that require
high temperatures above
80 °C for optimal growth.
in that it is mostly composed of neutral sugars (mainly
Glc), instead of sulphated GlcA81 (Supplementary infor-
mation S2 (table)). not only does H. salinarum have a
higher rate of glycosylation, it also has a higher diver-
sity of glycan species and produces an additional type
of polysaccharide, which is composed on average of
10 repeating branched pentasaccharide units that con-
tain acetylated or sulphated sugars82. The effect of such
an increase in cell-surface protein glycosylation is an
increase in the total negative charge of the cell surface,
enabling the formation of a more extensive hydrated
shell in H. salinarum in comparison with H. volcanii.
Known archaeal N-linked oligosaccharides are
diverse in terms of their glycan composition, even
between very closely related archaeal species52,65 (FIG. 4;
Supplementary information S2 (table)). This diversity
is concordant with there being a wide range of glycosyl-
transferases in archaeal genomes. Apart from Aeropyrum
pernix and Methanopyrus kandleri83,84, which do not con-
tain an AglB homologue, AglB is the only conserved pro-
tein in the N-glycosylation pathway that is present in all
archaea. In bacteria, N-glycosylation is considered to be
a rare event and is represented mainly in Campylobacter
spp., Helicobacter spp. and Desulphovibrio spp.85. Thus,
N-glycosylation apparently is far more common in
archaea than in bacteria.
O-glycosylation. During post-translational modification
by the O-glycosylation pathway, sugars are attached step-
wise to the hydroxyl groups of specific serine/threonine
residues of the nascent protein. The pili of pathogenic
bacteria, such as Neisseria spp.86, and the flagella of
the pathogen Campylobacter jejuni87 and Helicobacter
pylori88 contain O-linked glycans, which have a vital role
in their pathogenicity89,90. In contrast to N-glycosylation,
little is known about the archaeal O-glycosylation path-
ways, besides reports of O-linked glycosylation of the
S-layer proteins of H. salinarum55 and H. volcanii53, as
well as of cytochrome b558 of S. acidocaldarius91 (see
Supplementary information S2 (table)). However, many
extracellular archaeal proteins (including sugar binding
proteins from Thermococcales92 and Sulfolobales93,
extracellular enzymes and S-layer proteins) contain
serine/threonine-rich stretches that are adjacent to trans-
membrane domains that bear similarities to the S-layer
protein of H. salinarum, suggesting that their amino
acids might also be O-glycosylated.
Extracellular secretion of polysaccharides. In addition
to the glycosylation of extracellular proteins, archaea
have been reported to secrete polysaccharides into their
growth medium. Halobacterium mediterranei exopoly-
saccharides were the first exopolysaccharides to be puri-
fied from archaea and can reach high levels (3 mg/ml) in
the growth medium94. Archaeal extracellular polysac-
charides range from simple Man-only carbohydrates
like those from Thermococcus litoralis95 to more com-
plex ones in haloarchaea94,96,97 and Sulfolobales98, which
contain sulphated sugars including Glu, Man, galactose
and rhamnose. In bacteria, the secretion of polysaccha-
rides is connected to the initiation of biofilm formation99,
and has a fundamental role in the matrix that stabilizes
biofilms100. In T. litoralis, it has been shown that Man-
containing exopolysaccharides were present in films of
cells that were grown on filters95. In Sulfolobus solfataricus
surface attachment leads to the production of exopoly-
saccharides containing Glu, Glcnac, Man and galac-
tose.101. During biofilm formation by three Sulfolobus spp.
(S. acidocaldarius, S. solfataricus and Sulfolobus tokodaii)
the same extracellular sugars were secreted in a sequential
manner, enabling cells to establish tower-like structures
emanating from the first layer of cells102.
Cell surface structures
like bacteria, archaea possess many distinct cell surface
structures that enable them to move across, to sense and
to adhere to surfaces (FIG. 5). In recent years, archaeal
flagella and pili have been studied in more detail, and
it has been shown that a number of these structures are
assembled by systems that are similar to type IV pili
(which are important in bacterial motility, DnA uptake
Surface appendages and pili. Intriguing structures
known as cannulae have been found in strains of the
marine hyperthermophilic genus Pyrodictium103. As
Pyrodictium spp. cells divide, the two daughter cells
remain connected through cannulae, which can be
30–150 μm long104, leading to a dense network of cells
and tubules (FIG. 5). Cannulae are hollow tubes with a
diameter of 25 nm that seem empty when viewed in
cross section105. They enter the quasi-periplasmic space
of Pyrodictium spp. cells but do not seem to be connected
to the cytoplasm106. Therefore, it is unclear whether these
surface structures are used for the exchange of nutrients
between cells or whether they mainly provide a means
of attachment and anchoring.
Another novel archaeal surface structure is the hamus;
this was isolated from the euryarchaeon SM1 that grows
in cold (around 10 oC) sulphidic springs107,108. SM1 cells
have been found in mixed bacteria–archaea communi-
ties in a string-of-pearls arrangement in which the outer
layer is formed by the bacterial species and the inside of
the pearl by SM1. In these pearls, SM1 cells express up to
100 hami on their surfaces (FIG. 5). The hami are 7–8 nm
in diameter and are formed by a helical structure from
which three hooks emanate every 4 nm. At the distal end
of each hamus is a tripartite barbed grappling hook that
is 60 nm in diameter107. Similar to cannulae, hami are
thought to enable cells to attach to each other and there-
fore initiate community formation. Both cannulae and
hami are highly stable and resistant to high-temperature
treatment, enabling them to tether cells to surfaces even
under unfavourable conditions107,109. At present, there is
a lack of information about the protein make-up of hami
and cannulae and how they are assembled.
As yet, only one archaeal pilus has been described
that is not classified as a type IV pilus, namely the
5 nm diameter pilus of Methanothermobacter thermo
autotrophicus110. As with cannulae and hami, this pilus
mediates surface adhesion and cell–cell contacts in
biofilms of M. thermoautotrophicus.
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Part of a preprotein that
targets itself to the secretion
machinery in the cytoplasmic
Archaeal type IV pili. Flagellin, which is the structural
subunit of the archaeal flagellum, was the first prepilin
recognized to have similarities with bacterial type IV
pilins111. Bioinformatic predictions then resulted in the
identification of multiple putative pilins in archaeal
genomes112. These pilins have a genomic location close
to the PilT, PilB and PilC homologues, which are the
bacterial ATPases responsible for pilin disassembly and
assembly, and the central inner membrane protein of
bacterial type IV assembly machineries, respectively.
Therefore, it is likely that these proteins encode subunits
of archaeal surface structures. As in bacteria, archaeal
prepilins possess class III signal peptides (type IV pilin
signal peptides) that are processed before assembly by a
designated signal peptidase, which is called PilD in bac-
teria113. Archaeal PilD homologues have been mainly
studied in methanogens and Sulfolobales and are called
FlaK and PibD, respectively114,115. Both FlaK and PibD
are aspartyl-peptidases116,117, but their substrate specifi-
city differs in that the sole substrate of FlaK is preflagel-
lin, whereas PibD seems to recognize all class III signal
peptide-containing substrates tested so far (preflagellins,
prepilins and class III signal peptide-containing sugar-
binding proteins)114. In methanogens, another group
of prepilins was identified that belong to a conserved
archaeal group of hypothetical proteins (containing
DuF361 domains, which are present in genomes that
also encode EppA (a FlaK and PibD homologue112).
This peptidase has been shown to process the DuF361
domain-containing pilins in vitro112. Furthermore, it has
been shown that M. maripaludis does indeed express a
surface structure that contains three previously predicted
DuF361 domain-containing pilins60.
Pili with a diameter of 15 nm were identified in
Ignicoccus hospitalis, the prepilins for which also con-
tained type IV pilin signal peptides118. Interestingly, these
Figure 5 | Electron micrographs of different archaea possessing a range of surface appendages. a | A transmission
electron micrograph of negatively stained of Sulfolobales acidocaldarius cells showing flagella (~14 nm in diameter, red
arrows) pili (~10–12 nm, white arrows) and threads (~5 nm, black arrows). b | A scanning electron micrograph of
Methanocaldococcus villosus157 cells grown on a surface, exhibiting bundles of flagella that act in cell–cell connections and
surface adherence. c | Electron micrograph of a platinum-shadowed SM1 euryarchaeal coccus. d | Three-dimensional
model of the hamus structure as visualized by surface rendering of a de-noised data set, obtained by cryo-electron
tomography. The hook is 60 nm in width. e | Scanning electron micrograph of Pyrodictium spp. cells growing in a net of
cannulae. Image in part a courtesy of M. Ajon, University of Groningen, the Netherlands and S.-V.A.; image in part b
courtesy of G. Wanner, University of Munich, Germany; images in parts c and d reproduced, with permission, from ReF.107
© (2007) Wiley-Blackwell; image in part e reproduced, with permission, from ReF.109 © (1995) Elsevier.
422 | junE 2011 | VoluME 9
fibres were so brittle that they could only be observed in
cells that were not centrifuged before electron micros-
copy. At present, it remains to be determined whether
these pili are involved in attachment.
In Sulfolobales spp., especially S. acidocaldarius, a
range of different surface structures are present (FIG. 5).
Currently, there are two surface structures besides the
flagellum that have been characterized in Sulfolobales:
uV light-induced pili and the bindosome. In response
to DnA damage by uV light, S. solfataricus and S. acido
caldarius express proteins involved in DnA repair and also
upregulate gene expression from the uV light-induced
pili operon of Sulfolobales (ups), which encodes a hypo-
thetical protein, a PilT homologue, a PilG homologue
and two putative prepilins119–121. Electron microscopy
analysis showed that uV light-stressed S. solfataricus cells
have uV light-induced pili on their surfaces. These pili
are absent in a deletion mutant lacking the PilT homo-
logue of the highly induced ups operon. These pili were
also essential for uV light-induced cell aggregation121.
DnA double-strand breaks by agents such as bleomycin
or mitomycin triggers the induction of the ups operon,
which leads to the same aggregation and pili formation
that is seen in uV light-treated cells121. Therefore, this
system is thought to be involved in DnA exchange dur-
ing DnA repair. This system is present in all Sulfolobales
sequenced so far, so it will be interesting to investigate
whether it increases the fitness and resultant survival of
these strains in their natural environment.
In contrast to uV light-induced pili, bindosomes
were only found in the genomes of S. solfataricus and
the closely related S. islandicus. In these strains, a
number of sugar-binding proteins (Glc-, arabinose- and
trehalose-binding proteins) were identified as contain-
ing type IV pilin signal peptides that are processed by
PibD and do not contain more conventional secretory
signal peptides61,114,122. using mutational studies, a pilus
assembly system was identified that is required for the
functional localization of the sugar-binding proteins
to the S. solfataricus cell surface. This system contains
three pilin-like proteins, BasA, BasB and BasC, which
have an accessory role in this process. Deletion of basE
and basF (which are two of several pilT and pilG homo-
logues that are found in Sulfolobales, respectively)
resulted in impaired uptake and growth on sugars that
are transported by sugar-binding proteins containing
type IV pilin signal peptides123. Although a pilus-like
structure containing sugar-binding proteins has yet to
be shown, recent results suggest that such a structure
may be an integral part of the S. solfataricus S-layer.
There, it would contribute to the species’ extensively
lobed cell morphology and increase the cell surface
area, thus optimizing carbohydrate uptake124. Whereas
basE and basF deletion rendered S. solfataricus cells
completely round, complementation with basE and
basF restored the lobed morphology to a degree com-
parable with wild-type cells124. Bindosomes thus con-
tribute to the ability of S. solfataricus and S. islandicus
to metabolize a highly diverse set of carbohydrates and
might convey an important advantage in the native
The final archaeal type IV pilus discussed here is
the archaeal flagellum, which is the best studied of
archaeal surface structures (for excellent and detailed
reviews, see ReFS 125,126). In the 1980s it was shown
that H. salinarum harbours polar flagella that, like
those of bacteria, are used for swimming. In response
to extracellular stimuli such as blue light, these flagella
rotate and switch the direction of rotation from clock-
wise to anticlockwise127. These observations implied
that archaeal flagella and bacterial flagella share com-
mon features; for example, the use of type III secretion
systems during flagellar assembly, and the presence of
the membrane-bound flagellar motor consisting of a
multitude of subunits that enable rotation by use of the
proton motive force.
However, sequenced archaeal genomes revealed that
the assembly machineries of bacterial type IV pili — as
opposed to those of type IV flagella — are most related
to the archaeal flagellar assembly systems. like archaeal
prepilins, preflagellins are processed by PibD or FlaK and
then assembled by machinery containing FlaI and Flaj
(which are homologues of PilT and PilG, respectively).
Also in contrast to the bacterial flagellum, the archaeal
flagellum is not driven by the proton motive force and
instead relies on ATP hydrolysis — further supporting its
structural analogies to type IV pili128. operons encoding
genes for the archaeal flagellum usually include around
seven proteins in crenarchaeota and 10–15 proteins in
euryarchaeota. In H. salinarum, the additional euryar-
chaeal proteins are most probably involved in receiving
and transducing signals that can lead to a switch in the
rotational direction of the flagellum129. It has been dem-
onstrated in M. maripaludis130 that all Fla proteins are
necessary for proper assembly of the flagellum. only
limited data are available on the interactions of the Fla
proteins during assembly and the roles that different
subunits have in this process. Moreover, an important,
albeit unresolved, question is how a type IV pilus struc-
ture can generate enough torque to rotate a filament that
is responsible for swimming motility.
The functional and structural features of the archaeal
flagellum seem to indicate that the bacterial flagellum
is a truly bacterial structure and that type IV pili may
have been present in the common ancestor of Bacteria
and Archaea. The structure must have then evolved
divergently according to the needs of its host. Although
this assembly system has retained relative simplicity in
Archaea, the comparatively complicated bacterial cell
envelope must have brought about sequential changes
in the assembly machinery, thus resulting in modern
bacterial type IV pili systems.
Function of surface structures. Most surface structures
are either involved in cellular attachment to surfaces or
in initiating and/or maintaining cell–cell contacts. For
example, cannulae form during Pyrodictium spp. cell
division to ensure that daughter cells retain their prox-
imity to the parent cell and might also be used for the
exchange of nutrients103. Hami enable the euryarchae-
ote SM1 to adhere to stones in streams and engage in
mono-species community formation and dual-species
nATuRE REVIEWS | MicrobioLogy
VoluME 9 | junE 2011 | 423
biofilms108,131. As demonstrated in P. furiosus, flagella are
involved in cell–cell surface attachment in order to initi-
ate community formation and form cell–cell contacts132.
Although unimportant in initial biofilm formation in
static S. solfataricus cultures102, flagella are essential
for adherence in shaking cultures101. uV light-induced
cell aggregation in Sulfolobales was reported to involve
uV light-induced pili121 and, interestingly, a pilus
deletion mutant in S. solfataricus was unable to adhere
to a range of surfaces101 and biofilm formation was
Because PibD deletion mutants of H. volcanii lack all
type IV pili cell surface structures yet can still attach to
glass surfaces133, and swimming motility with flagella
has been reported for haloarchaea127 and P. furiosus132,
these surface appendages must have different roles in
different archaeal organisms despite striking similarities
in their assembled structures. Flagella and pili are used
by archaeal viruses as sites of attachment to host cells134,
and the incorporation of different variants of flagellin by
haloarchaea is suggested to be a defence mechanism for
preventing viral invasion135. In summary, the processes
in which surface structures are involved can vary even
between closely related archaea and seem to be very pre-
cisely adapted to the needs of each particular species in
order to enhance its survival in a specific habitat.
In the twentieth century, a number of initial studies were
performed on cell envelope components from archaea.
However, in contrast to research on the biogenesis of the
bacterial cell envelope, progress in elucidating pathways
for the biogenesis and function of the archaeal cell surface
has been sluggish. This is probably a result of the difficul-
ties in cultivating many known archaeal species and of the
poor availability of genetically tractable systems, which
have been developed for only a few archaeal species to
date. Recently, tremendous progress has been made, and
genetic systems now exist for four major euryarchaeal
and one crenarchaeal genus136, which are finally enabling
scientists to confirm protein functions in vivo.
In Archaea, three different cell division mechanisms
seem to exist: the bacterial-like FtsZ-based system, the
ESCRT-III based system (described for S. acidocaldar
ius137,138) and a putative system relying on archaeal actin-
like proteins139. In bacteria, peptidoglycan enzymes are
used for the positioning of the divisome to the site of
division24; however, for the archaeal cell division systems,
there is a lack of information about the remodelling of
the S-layer and of pseudomurein during cell division.
labelling of dividing archaeal cells with fluorescent dyes
showed that newly formed cell wall material is incorpo-
rated in a bacteria-like fashion140; however, the enzymes
involved in the biosynthesis of the cell wall material are
In view of the range of surface structures found in the
domain of Archaea — that is, glycans and other unusual
adaptations of the cell envelope to extreme environments
— the improved genetic tools available for archaea will
lead not only to the unravelling of new pathways but also
to the identification of enzymes with properties that may
have potential applications in biotechnology.
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forming cell-cell contacts. Int. J. Syst. Evol. Microbiol.
9 Jul 2010 (doi:10.1099/ijs.0.023663-0).
B.H.M. and S.-V.A. were supported by a VIDI grant of the
Dutch Science Organization (NWO) and S.-V.A. received addi-
tional intramural funds from the Max Planck Society. We want
to thank R. Rachel, C. Moissl and G. Wanner for providing us
with unpublished picture material. We thank A. Bozarth for
critical reading of the manuscript.
Competing interests statement
The authors declare no competing financial interests.
Sonja Alber’s homepage:
See online article: S1 (table) | S2 (table)
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426 | junE 2011 | VoluME 9
About the authors
Sonja-Verena Albers is a Max Planck Research Group leader at the
Max Planck Institute for Terrestrial Microbiology, Marburg, Germany.
Since receiving her Ph.D. from the university of Groningen, the
netherlands, she has been interested in the biogenesis of the archaeal
cell envelope focusing on the assembly and function of surface
appendages, Nglycosylation and the development of genetic tools
Benjamin H. Meyer studied biology at the Goethe university
Frankfurt, Germany, and is in the second year of his Ph.D. at the
Max Planck Institute for Terrestrial Microbiology, in the labora-
tory of Sonja-Verena Albers. He is involved in the elucidation of the
Nglycosylation pathway in the thermoacidophilic crenarchaeote
Online ‘at-a-glance’ summary
• The cell envelope of archaea is fundamentally different from bac-
teria in that it does not contain peptidoglycan, and archaeal mem-
branes are composed of ether lipids instead of ester lipids.
• Most archaea are surrounded by a surface-layer (S-layer), which
is a proteinaceous two-dimensional crystal layer. Some archaeal
cell envelopes contain pseudomurein or other unique sugar
• Most of the extracellular archaeal proteins are glycosylated
(N-linked, Olinked or both). The archaeal Nglycosylation path-
way bears similar features to both the eukaryotic and the bacterial
pathway. The known archaeal Nglycans are exceedingly diverse in
their composition and structure.
• Most archaeal pili and all archaeal flagella studied to date are assem-
bled by simple type IV pilin-like machineries.
Table of contents
000 The archaeal cell envelope
Sonja-Verena Albers and Benjamin H. Meyer
The archaeal cell surface is home to a range of lipids,
proteins, polysaccharides and surface structures
that are distinct from those observed at the bacterial
cell surface. In this Review, Albers and Meyer discuss
our current understanding of the composition of the
archaeal cell envelope.