The dynamic polymerization of actin has a central role
in several processes that reshape the plasma membrane.
These processes include the protrusion of lamellipodia
and filopodia during cell migration, and different forms
of endocytic internalization — for example, phagocyto-
sis, macropinocytosis, clathrin-mediated endocytosis and
caveolae-mediated endocytosis (FIG. 1). In this review,
we focus on the role of actin in clathrin-mediated
endocytosis, which is the main pathway for receptor-
mediated endocytosis in most eukaryotic cells1 (for
other reviews on actin in clathrin-mediated endocyto-
sis, see REFS 2–5). We present recent findings on this
topic with an emphasis on live-cell imaging studies in
Saccharomyces cerevisiae, in which the analysis of actin
dynamics and function has been most comprehensive.
We also attempt to synthesize all of the findings into
a mechanistic model of how actin functions during
A basic scheme for the key events of clathrin-mediated
endocytosis was established decades ago6–8. The first
step in this pathway is the binding of extracellular
cargo molecules to specific cell-surface receptors.
These receptors and other membrane proteins that
are destined for endocytosis are then sequestered by
intracellular adaptor proteins to sites of endocytosis.
The adaptors, together with clathrin, form an endocytic
coat at the plasma membrane. The coated membrane
bends to form an invagination that subsequently
pinches off to form a cargo-filled vesicle. The protein
coat that surrounds the newly formed endocytic vesi-
cle is rapidly disassembled. The vesicles then deliver
their cargoes to early endosomes by fusing with the
endosomal membrane. From early endosomes, the
cargo molecules can be recycled back to the plasma
membrane or trafficked further to late endosomes
and, finally, to lysosomal organelles for degradation.
This classic model was based mainly on the results
of electron-microscopy studies on fixed cells and on
numerous biochemical assays.
During recent years, live-cell fluorescence micro-
scopy has provided several new mechanistic insights into
clathrin-mediated endocytosis. Although the resolution
of light microscopy cannot compete with that of electron
microscopy, light microscopy has one important advan-
tage — microscopy at visual wavelengths can be carried
out on living cells, so cellular processes can be followed
in real time. Owing to the transient and localized nature
of endocytic actin structures, live-cell imaging has been
crucial in showing that a dynamic actin cytoskeleton
participates directly in membrane dynamics during
clathrin-mediated endocytosis and in understanding
the role of actin in endocytosis. The dynamic connec-
tion between the actin cytoskeleton and sites of clathrin-
mediated endocytosis in living cells was first shown
in cultured mammalian cells9. As described below,
the polymerization of actin seems to provide the force
for the deformation and movement of a membrane at
different steps along the endocytic pathway.
Linking the endocytic and actin machineries
One of the first hints that the actin cytoskeleton is
involved in endocytosis came from the use of pharma-
cological agents to interfere with actin turnover. In
mammalian cells, actin poisons inhibit endocytic uptake
and the formation of coated vesicles10–13. However, the
block in endocytosis in mammalian cells seems to be
partial10,12,13 or restricted to only the apical surface of epi-
thelial cells11. In S. cerevisiae, both latrunculin A, which
binds to actin monomers and prevents their assembly
into filaments, and jasplakinolide, which stabilizes actin
filaments (and prevents their depolymerization), block
Department of Molecular and
Cell Biology, University of
California 94720-3202, USA.
Correspondence to D.G.D.
protrusions at the leading edge
of motile cells that are formed
by actin polymerization.
Plasma-membrane spikes that
are formed by actin
associated process in which a
eukaryotic cell engulfs large
particles, such as bacteria.
A form of endocytosis in which
extracellular fluid is taken up
through the formation of large
The uptake of receptors,
membrane and cargo at the
cell surface through a process
that specifically involves the
coat protein clathrin.
Harnessing actin dynamics for
Marko Kaksonen, Christopher P. Toret and David G. Drubin
Actin polymerization often occurs at the plasma membrane to drive the protrusion of
lamellipodia and filopodia at the leading edge of migrating cells. A role for actin
polymerization in another cellular process that involves the reshaping of the plasma
membrane — namely endocytosis — has recently been established. Live-cell imaging studies
are shedding light on the order and timing of the molecular events and mechanisms of actin
function during endocytosis.
404 | JUNE 2006 | VOLUME 7
endocytosis (actin patch)
A form of uptake at the plasma
membrane that involves the
A conserved actin-binding
protein that is thought to be
involved in actin-filament
severing and disassembly.
Another line of evidence for the involvement of actin
in endocytosis came from genetic experiments that were
carried out in S. cerevisiae. Using actin mutants, the
Riezman laboratory showed that normal actin function
is required for the internalization step of the endocyto-
sis of mating pheromone α-factor16. The later trafficking
steps along the endocytic pathway were not affected16.
Similarly, mutations in several actin-binding proteins
resulted in a block in endocytic internalization in
S. cerevisiae. In cells in which the gene that encodes the
actin-filament-crosslinking protein Sac6 (also known as
fimbrin) was deleted, the internalization of α-factor was
blocked16 (see TABLE 1 for descriptions and homologies
of the S. cerevisiae and mammalian proteins that are dis-
cussed in this article). Deletions of the genes that encode
the S. cerevisiae type-I myosins Myo3 and Myo5, which
are actin-dependent motor proteins, also caused a strong
defect in receptor-mediated endocytosis17. In addition,
cofilin mutations and the actin Val159Asn mutation,
which both cause defects in actin-filament turnover,
blocked the endocytic uptake of Lucifer yellow. These
results indicate that active actin-filament assembly
and disassembly are required for endocytosis.
The localization of actin also indicates that it might
function in endocytosis. In S. cerevisiae cells, punc-
tate cortical actin structures called actin patches were
shown to colocalize, albeit only partially, with many
endocytic proteins2. Also, electron-microscopy stud-
ies on mammalian cells showed that actin filaments
were often associated with coated pits13. In neuronal
synapses, actin was also found to colocalize with sites of
clathrin-mediated endocytosis18. Similarly, an immuno-
electron-microscopy study found that S. cerevisiae actin
patches localized to membrane invaginations that could
be intermediates of endocytic vesicle formation19. How-
ever, a follow-up study, which used double immuno-
electron microscopy, failed to show the colocalization of
endocytic cargo molecules with actin patches20.
Finally, an abundance of studies have uncovered a
large number of protein–protein interactions that are
indicative of links between the endocytic machinery
and the actin cytoskeleton. These studies of biochemical
interactions show that many endocytic proteins can
potentially be linked to the actin cytoskeleton, either
directly or indirectly2,5,21. Taken together, these experi-
ments clearly indicate a close functional link between
the actin cytoskeleton and the internalization step of
endocytosis. However, an understanding of how actin
functions in endocytosis remained elusive.
Glimpses of actin in living cells
Live-cell imaging studies of both mammalian and
S. cerevisiae cells have recently revealed a highly regular
timeline of events at endocytic sites. Different endocytic
proteins are recruited to and disassemble from endo-
cytic sites in an ordered sequence9,22–31. Many features
of this sequence are conserved between mammalian and
S. cerevisiae cells, as are the proteins themselves (FIG. 2;
In mammalian cells, actin appears at clathrin-coated
structures on the plasma membrane in transient bursts
that occur near the end of the lifetime of the clathrin-
coated pit and overlap with the internalization of
clathrin-coated vesicles9,32 (FIG. 2). The actin structures
that are involved in these bursts have been visualized
in mammalian cells by expressing green fluorescent
protein (GFP) fusions of actin or by microinjecting
fluorescently labelled actin, and using total internal
reflection fluorescence (TIRF) microscopy (BOX 1).
Internalization is marked by the movement of fluo-
rescently labelled clathrin away from the cell surface
and can be followed by comparing the signal intensi-
ties in TIRF and epifluorescence images9. This ratio
Figure 1 | Actin-polymerization-driven processes in eukaryotic cells. The dynamic
polymerization of actin filaments (red) is involved in different processes that reshape or
move cellular membranes. These processes include different forms of endocytic uptake
at the plasma membrane — that is, clathrin-mediated endocytosis in Saccharomyces
cerevisiae and mammalian cells, as well as caveolae-mediated endocytosis,
macropinocytosis and phagocytosis in mammalian cells. In addition, actin assembly has
a role in the movement of endosomes and/or endocytic vesicles. In mammalian cells,
endosomes move by actin ‘rocketing’, whereas in S. cerevisiae, endocytic vesicles move
together with actin cables as they are being assembled by formin proteins. Finally, the
protrusion of lamellipodia and filopodia in migrating mammalian cells is dependent on
NATURE REVIEWS | MOLECULAR CELL BIOLOGY
VOLUME 7 | JUNE 2006 | 405
A fluorescent dye that enters
cells by endocytosis and is
often used as a marker for bulk
(intensity in TIRF/ intensity in epifluorescence) decre-
ases when the fluorescent structures move into the cell
and out of the zone of TIRF excitation9. The increase
in the intensity of actin fluorescence correlates with
the translocation of the clathrin signal away from the
plasma membrane9. Merrifield and colleagues reported
that bursts in actin polymerization occurred at 80% of
the internalizing clathrin-coated pits9. Internalization
also coincides with the transient recruitment of
dynamin-1, a GTPase that is involved in vesicle scis-
sion9. Actin polymerization is therefore tightly coupled
both spatially and temporally to the vesicle-budding
step of endocytosis.
A similar sequence of events takes place at endo-
cytic sites in S. cerevisiae (FIG. 2). The two main actin
cytoskeletal structures in S. cerevisiae are actin cables
and actin patches (FIG. 1). The cables are orientated along
the growth axis of the cell and are involved in the traf-
ficking of many organelles to growing daughter cells33.
Actin patches are punctate dynamic structures that are
at or close to the plasma membrane34,35. The dynamics
of actin patches have been followed using different
fluorescently tagged actin-cytoskeleton proteins23–25,34–39.
For a long time the role of actin patches remained
unknown. However, it has recently become evident
that they mark endocytic sites at the plasma membrane
and newly formed primary endocytic vesicles24,25,28,37.
Another long-standing issue in the studies of endo-
cytosis in S. cerevisiae has been the unresolved role of
clathrin. The question of whether clathrin localizes to
endocytic sites at the plasma membrane in S. cerevisiae
has been difficult to answer owing to technical chal-
lenges. However, using TIRF microscopy, it has recently
been shown that clathrin does localize to S. cerevisiae
endocytic sites, as was expected from work in other
species, and that actin polymerization bursts occur
at the clathrin-containing endocytic sites during the
vesicle-budding phase25,28. Actin patches are highly
transient, with lifetimes of only about 15 seconds24,39.
Similar to the case in mammalian cells, these transient
actin structures form at the plasma membrane, where
they colocalize with endocytic sites that are marked
with clathrin and other endocytic proteins25,28. The short
lifetime of the actin patches is divided into an initial
phase of restricted motility followed by a phase of rapid
motility during which the patch disassembles24. The
initial phase probably coincides with the internalization
movement of clathrin-coated endocytic structures25 and
the rapid motility phase probably corresponds to the
movement of clathrin-uncoated, actin-filament-covered
The localization and timing of the actin polymeriza-
tion bursts are strikingly similar in both S. cerevisiae and
mammalian cells (FIG. 2). Furthermore, the spatial and
Table 1 | Saccharomyces cerevisiae and mammalian protein homologies*
ABP1 Arp2/3-complex activator in S. cerevisiae, actin-binding
protein, scaffold protein
Barbed-end actin-filament capping proteins
(Pan1, Sla1, End3)
*This table gives descriptions and homologies of the S. cerevisiae and mammalian proteins that are discussed in this article. The ‘–’
symbol indicates that there is no known homologue. AAK1, adaptor-protein-complex-2-associated kinase-1; Abp1/ABP1, actin-binding
protein-1; Ark1, actin-regulating kinase-1; Arp2/3, actin-related protein-2/3; Cap 1/2, barbed-end capping proteins; CAPZ, capping
protein muscle Z-line; Chc1, clathrin heavy chain-1; Clc1, clathrin light chain-1; EPS15, epidermal-growth-factor-receptor-pathway
substrate-15; GAK, cyclin-G-associated kinase; HIP1R, Huntingtin-interacting protein-1 related; Myo, myosin; PtdIns(4,5)P2, phosphati-
dylinositol-4,5-bisphosphate; Rvs, reduced viabililty upon starvation; (N-)WASP, (neuronal) Wiskott–Aldrich syndrome protein; WIP,
Vesicle coat component
Arp2/3-complex activator in S. cerevisiae, motor protein
Adaptor protein, Arp2/3-complex activator (Pan1 in
S. cerevisiae), WASP regulator (Sla1, Intersectin)
Membrane bending or curvature sensing
Actin-binding protein, PtdIns(4,5)P2-binding protein
GTPase, vesicle scission
406 | JUNE 2006 | VOLUME 7
Endocytic site initiationInvagination and scission Release
b Saccharomyces cerevisiae cells
c Mammalian cells
Clathrin (1–2 min)
Actin, Arp2/3, Cortactin
Sla1, Pan1, End3, Sla2 (30–40 s)
Las17 (30–40 s)
Myo5, Bbc1 (10 s)
Actin, Arp2/3 complex, Abp1 Cap1/2, Sac6 (15 s)
Rvs161, Rvs167 (10 s)
A protein complex that
promotes the nucleation of
actin filaments and creates a
branched actin meshwork.
A family of proteins that
contain a formin homology-2
domain and can promote
temporal localization of several endocytic and actin-
cytoskeleton proteins has been determined in both
S. cerevisiae and mammalian cells, and it is apparent that
the endocytic internalization processes in these different
organisms are variations on the same ancestral theme
(FIG. 2). However, there are also striking differences,
such as the importance of dynamin. In mammalian
cells, dynamin is essential for the scission of clathrin-
coated vesicles41,42. By contrast, dynamins do not seem
to have a direct role in endocytic internalization in
S. cerevisiae43,44. Also, the relative importance of clathrin
and adaptor protein complex-2 (AP2) seems greater
in mammalian than in S. cerevisiae cells.
Dynamic actin filaments have also been observed
to colocalize with endosomes in cells from various
vertebrate species45–49 (FIG. 1). Actin ‘comet tails’ form on
one side of the endosomes and actin-filament assembly
in the comet tails seems to push the endosomes along.
The biological role of the actin rocketing of endocytic
organelles is not clear. It might have a role in moving the
endosomes so that they can make contact with micro-
tubules, which are used as tracks for long-range traffick-
ing, or it could aid in the fusion of endocytic organelles.
A similar actin-comet-tail-propelled movement has also
been proposed as a mechanism for endosome motility
in S. cerevisiae cells50. However, no actin comet tails or
actin-related protein-2/3 (Arp2/3)-complex activators
have been observed to associate with S. cerevisiae endo-
somes, so whether this mechanism exists in S. cerevisiae
remains an open question.
S. cerevisiae cells use a different actin-polymerization-
dependent mechanism for endocytic vesicle motility.
Actin cables are bundles of actin filaments that grow
continuously by polymerization that is nucleated at the
plasma membrane by formin-family proteins51. Endocytic
vesicles become attached to the cables and move
together with the growing cables37,52 (FIG. 1). This process
seems to improve the the efficiency of the trafficking of
In summary, actin polymerization seems to have a
direct role during at least two steps in the endocytic
pathway. First, actin appears at the plasma membrane
during the formation of the primary endocytic vesicles.
Subsequently, in some vertebrate cells, dynamic actin
structures associate with motile endosomes.
The roles of actin in endocytic internalization
Dynamic actin structures seem to associate with dif-
ferent endocytic intermediates. But what are the func-
tional roles of actin during endocytosis? Actin could
function during several different stages of clathrin-
mediated endocytosis (FIG. 2). In both mammalian
and S. cerevisiae cells, the first step of endocytosis in
which dynamic actin structures has been visualized
is just prior to the internalization of the endocytic
coat9,24. This timing indicates possible roles for actin
in membrane invagination, vesicle constriction, vesicle
scission and vesicle movement. Actin might also have
earlier, possibly indirect, roles in the spatial organiza-
tion and lateral movement of clathrin-coated endocytic
sites32,53,54. It was shown recently in mammalian cells
that vesicle scission takes place when the actin poly-
merization burst reaches its peak level26. Merrifield
and colleagues used mouse fibroblast cells that were
expressing a pH-sensitive form of GFP that was fused
to the extracellular domain of the transferrin receptor.
The cells were perfused with rapidly alternating media
of neutral and low pH, and the surface-exposed recep-
tors changed their fluorescence intensity depending on
the pH of the medium. However, when the receptors
were internalized they became protected from the pH
changes. This method allowed vesicle scission events to
be observed in relation to actin and clathrin dynamics
at a time resolution of 2 seconds26. The exact timing of
Figure 2 | The sequential assembly of proteins at endocytic sites. a | The different
steps of endocytic internalization: endocytic site initiation, membrane invagination and
scission, and vesicle release. The four protein modules that are involved in endocytic
internalization in Saccharomyces cerevisiae are shown schematically — that is, the coat
(green), the Wiskott–Aldrich syndrome protein (WASP)–myosin complex (yellow), the
actin network (red) and the amphiphysin complex (blue). Components of these different
protein modules are assembled and disassembled dynamically. b | The temporal
localization of the constituent proteins for each module in S. cerevisiae23–25. c | The
approximate temporal localization of proteins during endocytic internalization in
mammalian cells9, 27. Dashed lines indicate ambiguity in the time frame of the protein
dynamics. Endocytic protein modules have not been defined for mammalian cells. Abp1,
actin-binding protein-1; Arp2/3, actin-related protein-2/3; Cap1/2, barbed-end capping
proteins; Myo, myosin; N-WASP, neuronal WASP ; Rvs, reduced viability upon starvation.
For further information on the proteins involved, see TABLE 1.
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vesicle scission has not been determined in S. cerevisiae.
However, the fact that the S. cerevisiae actin patches
enter their phase of rapid motility briefly after maximal
actin accumulation occurs indicates that the timing of
vesicle scission in relation to actin-filament assembly
is similar in both mammalian and S. cerevisiae cells24.
Actin polymerization therefore seems to begin when
the coat and the plasma membrane start to invaginate
to form a coated pit, and net actin polymerization
seems to stop when vesicle scission occurs. This timing
indicates that actin polymerization could have roles in
invagination, constriction and scission.
Experimental perturbations of actin function pro-
vide important insights into the endocytic roles of actin.
In S. cerevisiae, when actin polymerization is blocked
by latrunculin A, the clathrin coats and coat-associated
proteins are stabilized at the plasma membrane and the
internalization movement is completely blocked24,25,28.
Similarly, coat internalization is blocked by certain
Arp2/3-complex mutants55. These results indicate that
actin polymerization in S. cerevisiae is needed for the
initial formation of invaginated coated pits.
In mammalian cells, the scission of clathrin-coated
vesicles from the plasma membrane was reduced by
82% in cells that were treated with the inhibitor of actin
polymerization latrunculin B (REF. 26). However, whether
endocytic invaginations were formed was not deter-
mined in this study. Using electron microscopy, Yarar
and colleagues showed that although endocytosis was
inhibited in cells that were treated with latrunculin A
or jasplakinolide, the number of invaginated coated
pits was actually higher in treated cells than in control
cells32. This indicates that invaginations form without
actin function in mammalian cells but that efficient
vesicle scission requires actin. Also, constriction of
the vesicle neck was impaired when actin function
was perturbed32. By contrast, S. cerevisiae cells seem
to need actin for membrane invagination preceding
vesicle scission24. This apparent difference between
S. cerevisiae and mammalian cells could be due to dif-
ferences in membrane tension. S. cerevisiae might need
more force to induce plasma-membrane invagination
owing to the high osmotic pressure in these cells that
results in high plasma-membrane tension. By contrast,
in mammalian cells, vesicle scission might be the step
that is most dependent on the forces that are provided by
actin-filament assembly. One fact that might complicate
the interpretation of results is that mammalian cells use
the actin cytoskeleton to maintain plasma-membrane
tension56. When the actin cytoskeleton is perturbed
globally in the cell, reduced membrane tension might
facilitate the formation of invaginations and explain the
reduced need for actin during this step.
Mechanics of actin-driven internalization
Certain intracellular bacterial and viral pathogens
such as Listeria, Shigella and Vaccinia spp. use an actin-
rocketing mechanism for their motility inside mamma-
lian host cells57,58. Brownian-ratchet models show that
actin polymerization alone can generate force to move
objects59 (BOX 2). This kind of polymerization-driven
mechanism has been proposed to be responsible for the
motility of pathogens, such as those mentioned above,
in the cytoplasm of infected mammalian cells (FIG. 3a)
and for the protrusion of the leading edge of migrating
cells (FIG. 3b). The actin-polymerization-driven motility
of bacteria, coated plastic microbeads and lipid vesicles
has also been observed in cytoplasmic extracts47,60–63,
and has been reconstituted using purified proteins64.
Only actin, the Arp2/3 complex, cofilin and capping pro-
tein are needed for this type of motility in vitro64. Proteins
at the surface of the propelled object (for example, the
Listeria monocytogenes surface protein ActA) activate
the Arp2/3 complex. Activated Arp2/3 complexes bind
to the sides of actin filaments and activate the nucleation of
new filaments. This leads to the formation of comet-tail-
like actin structures that are composed of branched
actin-filament networks. The comet tail grows at the
surface of the object and ‘rockets’ it forward.
This actin-rocketing model can explain the motility
of endosomes that have actin tails45–49. The requirements
are simply that the organelle nucleates actin-filament
assembly at its surface and that polymerization takes
Box 1 | Total internal reflection fluorescence microscopy
One of the common problems in imaging studies of membrane-trafficking events is that it
is often difficult to resolve different structures. Typical mammalian cells contain many
different kinds of actin structures that are used at various locations in a cell for structural
support, motility and trafficking. Similarly, clathrin localizes to the plasma membrane and
to internal organelles such as endosomes and the Golgi complex. Total internal reflection
fluorescence (TIRF) microscopy has been useful in studies of endocytosis because it
allows researchers to excite fluorescent molecules specifically at the interface between
the sample and the coverglass. In TIRF microscopy, the fluorescent molecules are excited
with a beam of light (usually from a laser) that is directed at the sample at an angle that is
greater than the critical angle such that all of the light is reflected at the coverglass–
sample interface. Although no light gets into the sample, an evanescent wave is created
on the sample side. This wave dissipates rapidly as the distance from the surface of the
coverglass increases. The evanescent wave excites fluorophores that are within ~200 nm
of the coverglass surface (the structures highlighted in red (actin) and green (clathrin) in
the figure, but not those that are further away (the structures with the same shape, but
that are coloured grey)). TIRF has been used to reveal the actin polymerization bursts at
endocytic sites in mammalian cells9,26,32 and the localization of clathrin at endocytic sites
in Saccharomyces cerevisiae cells25,28. TGN, trans-Golgi network.
408 | JUNE 2006 | VOLUME 7
a Brownian rachet
b Elastic Brownian rachet
place asymmetrically on the organelle (FIG. 3a). This
asymmetry has been shown to arise spontaneously in
an in vitro system that used plastic microbeads that were
coated with an Arp2/3-complex activator 65.
However, the rocketing model that has been seen in
Listeria spp. might not be applicable at sites of vesicle
formation at the plasma membrane. If actin filaments
are nucleated at an endocytic site and form an actin tail,
the resulting force would actually oppose membrane
invagination (FIG. 3c). The membrane would need to be
significantly invaginated prior to the initiation of actin-
filament assembly, and the actin filaments would need
to be orientated so that the force would be directed into
the cell (FIG. 3d), or orientated to constrict the neck of the
invagination to help the vesicle scission event. Different
kinds of schematic models have been proposed2,5, but
detailed information about the organization and polar-
ity of the actin filaments at endocytic sites is lacking.
Interestingly, an S. cerevisiae strain that carries deletion
mutations in two genes that are linked to actin regulation
has provided insights into actin organization. Cells that
lack the genes BBC1 and SLA1 can still carry out endo-
cytosis, but they have highly enlarged actin structures
at endocytic sites25. The large size of the actin structures
made it possible to use photobleaching studies to analyse
the turnover of actin during endocytic internalization.
These experiments showed that actin polymerizes at the
plasma membrane, and that the clathrin coat and the
invaginating membrane move into the cell at the same
rate as the growing actin network25.
On the basis of these findings and the dynamics
of proteins in wild-type cells, a new model for actin-
driven endocytosis has been proposed24,25 (FIGS 3e,4). In
this model, the endocytic coat is initially surrounded
by a complex of proteins (including Myo5 and Las17, a
Wiskott–Aldrich syndrome protein (WASP); TABLE 1)
that remains on the plasma membrane at the rim of
the invaginating endocytic pit and efficiently activates the
Arp2/3 complex to nucleate actin filaments. These fila-
ments then bind to the endocytic coat and continued
nucleation leads to the formation of a cone of crosslinked
actin filaments that pulls the attached coat inwards and
invaginates the underlying membrane19,66.
This model is supported by both localization data
and deletion phenotypes for many endocytic proteins.
The model assumes that the vesicle coat can bind to
actin filaments. Candidate proteins for linking the coat
to the actin meshwork include S. cerevisiae Sla2 and its
mammalian homologue HIP1R (Huntingtin-interacting
protein-1 related). These proteins can bind directly to both
actin filaments and clathrin, and they colocalize with the
clathrin coat67–69. Deletion of the S. cerevisiae SLA2 gene
and knockdown of HIP1R expression lead to a dramatic
phenotype in which actin polymerization is uncoupled
Box 2 | Brownian-ratchet models
Brownian-ratchet models provide mechanisms by which actin polymerization can drive the motility of ‘loads’ such as the
leading edge of migrating cells, endosomes or pathogenic bacteria that have invaded mammalian cells59,102,103. In the
original Brownian-ratchet model (see figure, part a), the actin filament (red) grows through the addition of ATP–actin
monomers towards the load, until it is so close to the load that there is no more space for the addition of further
monomers. However, if the load undergoes Brownian motion, it will occasionally move far enough away from the filament
end to allow a new monomer to be added to the filament. The continuing growth of the actin filament biases the Brownian
movement of the load in the direction of the filament growth.
The elastic Brownian-ratchet model (see figure, part b) is a modification of the original model. In the elastic model, the
random bending of the actin filament provides the space for monomer addition. The elastic energy that is stored in the
bent filament then pushes the load forward.
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Listeria or endosome
a Listeria and endosome rocketing
c Endocytic internalization: model 1
e Endocytic internalization: model 3
d Endocytic internalization: model 2
from the internalization of the vesicle coat24,70. Another
protein that can potentially link actin filaments to the
coat is S. cerevisiae Pan1, which shares homology with
the mammalian endocytic proteins EPS15 (epidermal-
growth-factor-receptor-pathway substrate-15) and inter-
sectin. Pan1 binds to actin filaments and colocalizes with
the clathrin coat25,71. Furthermore, Sac6 is essential for
internalization, but not for actin polymerization25, which
indicates that the integrity of the actin-filament network
is important for force transduction (FIG. 4).
In S. cerevisiae, the type-I myosins Myo3 and Myo5
are also important for endocytosis17, and actin-patch
internalization is severely defective in mutants of
these myosins23,39. The myosins remain localized at the
plasma membrane with Las17 while the coat proteins
are internalized (FIG. 4), and the timing of their appear-
ance at endocytic sites corresponds to the onset of
actin polymerization23. A similar sequence of recruit-
ment of a type-I myosin and a WASP homologue
has also recently been described in the fission yeast
Figure 3 | Modelling actin-driven endocytic internalization. a | An actin-rocketing model can explain the motility of
endosomes and intracellular pathogens such as Listeria species. Actin-related protein-2/3 (Arp2/3)-complex activators
are asymmetrically associated with the surface of the object being moved. Arp2/3-complex-nucleated filaments form an
actin ‘comet tail’ that is composed of branched filaments. b | The protrusion of a lamellipodium is also driven by actin
polymerization. Arp2/3-complex activators , such as WAVE proteins (Wiskott–Aldrich syndrome protein-family verprolin-
homologous proteins), localize to the leading edge and activate the Arp2/3 complexes there. The growing branched
network flows away from the leading edge. However, when the growing actin network is bound to focal-adhesion
complexes that are connected to the extracellular matrix, the leading edge is pushed outwards. c | An actin-rocketing
model in which the filaments are nucleated by the vesicle coat cannot explain endocytic internalization. If the vesicle coat
contained Arp2/3-complex activators, the forming actin comet tail would generate a force that would oppose
internalization. d | An alternative model requires that the plasma membrane has been bent to form a coated pit before
actin polymerization is initiated. Actin is polymerized at the rim of the coated pit to form a network of filaments that
pushes the coat towards the cell centre. e | In a third model, actin polymerization is nucleated by Arp2/3-complex
activators that surround the coated membrane, and actin filaments form a cone that surrounds the coat. The coat binds
to the filaments. The coat and the underlying membrane are pulled towards the interior of the cell by the movement of the
growing actin network.
410 | JUNE 2006 | VOLUME 7
Schizosaccharomyces pombe30. The myosin motor
domains could push the growing actin filaments away
from the membrane, or they could transiently anchor the
growing barbed ends to the plasma membrane. However,
S. cerevisiae type-I myosins can also activate the Arp2/3
complex72,73. Further work is needed to clarify the
relative contributions of myosin motor activity and
actin polymerization to endocytic internalization. There
is currently no evidence of a role for type-I myosins in
the vesicle-budding step in mammalian cells.
Controlling the actin engine
The actin polymerization that is associated with endo-
cytic processes seems to be tightly regulated. This is
especially evident at endocytic sites on the plasma
membrane, where transient bursts of actin polymeriza-
tion are precisely coordinated with the recruitment of
other endocytic proteins9,23–28,30. Actin polymerization
is initiated at preformed clathrin-coated structures
and stops rapidly after vesicle scission26. How is this
regulation achieved? The main control point for actin
polymerization is the Arp2/3-complex-mediated
nucleation event, which is the rate-limiting step in actin
polymerization74. In mammalian cells, the Arp2/3-
complex activators N-WASP and cortactin are known
to localize to the sites of vesicle budding27,75. Interfering
with N-WASP or cortactin function impairs receptor-
mediated endocytosis75–77. In S. cerevisiae, there are up to
five potential Arp2/3-complex activators that localize to
endocytic sites — Las17, Pan1, Myo3, Myo5 and actin-
binding protein-1 (Abp1)23,24,30. The dynamics of four
of these proteins (Las17, Pan1, Myo5 and Abp1) have
been studied in detail in living cells. Surprisingly, each
shows a different motile behaviour, which indicates that
they might harness the force of actin polymerization at
different stages of vesicle formation. Mutants of Las17,
Pan1, Myo3 and Myo5 cause severe defects in actin
organization and endocytosis17,78,79. Abp1 does not have
a significant role in initiating actin polymerization, but
seems to have an inhibitory role in that it turns off actin
The regulatory complexity of endocytosis does
not end here. The Arp2/3-complex activators are
themselves subject to regulation by many interacting
proteins. Mammalian N-WASP might be regulated
during the endocytic internalization process by ABI1
(Abl-interacting protein-1), syndapins, intersectin and
CDC42 (REFS 77,81–83). In S. cerevisiae, Las17 can be
regulated by Sla1, Bbc1 and Vrp1 (REFS 78,84). Vrp1 and
Sla1 also interact with Myo5 and Pan1, respectively 85,86.
Deletions of the genes that encode Sla1, Bbc1 and Vrp1
cause different endocytic phenotypes25. First, the dele-
tion of SLA1 leads to a delay in the initiation of actin
polymerization at endocytic sites25. Second, the dele-
tion of BBC1 causes an enhancement of the actin-driven
internalization movement25. Third, the deletion of VRP1
uncouples actin polymerization from the internaliza-
tion movement25. Protein phosphorylation also has a
role in regulating the actin polymerization bursts at the
internalization sites. The S. cerevisiae actin-regulating
kinases Ark1 and Prk1 are involved in turning off the
actin-polymerization machinery at the newly formed
endocytic vesicles. Inhibiting the kinases Ark1 and
Prk1 leads to the accumulation of clusters of actin-
filament-covered endocytic vesicles in S. cerevisiae
cells25,40,87. These kinases function at least in part by
phosphorylating Pan1 and inhibiting its function71,88.
Interestingly, Abp1 recruits the kinases Ark1 and Prk1
to endocytic sites87. In addition, Abp1 recruits the
S. cerevisiae synaptojanin Inp52, which is a phosphati-
dylinositol-4,5-bisphosphate 5-phosphatase that can
also negatively regulate actin-filament assembly89. The
localization of Abp1 depends on actin filaments15, so
Abp1 might function in a negative feedback loop that
is triggered by nascent actin filaments and functions to
turn off actin polymerization.
In mammalian cells, dynamin potentially has an
important role in coordinating actin-filament assembly
at endocytic sites. Dynamin is thought to oligomerize
around the neck of an endocytic membrane invagination
Figure 4 | Current model for actin-driven endocytic internalization. This schematic
diagram illustrates putative functions of different actin-cytoskeleton proteins during
endocytic internalization in Saccharomyces cerevisiae. Las17 ( Wiskott–Aldrich syndrome
protein (WASP) in mammals) together with the myosins Myo3 (not shown) and Myo5
activate the actin-related protein-2/3 (Arp2/3) complex at the cell surface. Myosins
might also generate force on the actin network or anchor the actin filaments to the
plasma membrane through their motor domains. The activated Arp2/3 complexes form
branched actin filaments that grow through the addition of ATP–actin monomers near
the plasma membrane. Older filaments are capped at their barbed ends by capping
proteins (Cap1/2). The branched filaments are further crosslinked by Sac6.
The crosslinked actin network is linked to the underlying vesicle coat by actin-binding
proteins such as Sla2 and Pan1, which are represented by green hand-like structures .
The growth of the actin network leads to the invagination of the coated membrane.
For further information on the proteins involved, see TABLE 1.
NATURE REVIEWS | MOLECULAR CELL BIOLOGY
VOLUME 7 | JUNE 2006 | 411
and facilitate vesicle scission, and it has been proposed
to function either by providing force for the constriction
of the neck or by functioning as a regu latory GTPase41,42.
The peak accumulation of dynamin-1 at endocytic sites
precedes peak actin accumulation9 (FIG. 2c). Dynamin
can interact with the actin machinery through several
linking proteins such as cortactin, intersectin and syn-
dapin90–92. These proteins bind to dynamin, the Arp2/3
complex, actin filaments and N-WASP. Dynamin might
regulate actin-filament assembly at endocytic sites,
coordinating it with the vesicle scission event. Dynamin
might also be linked to the actin cytoskeleton through
a family of proteins that contain BAR or related F-BAR
domains, which both bind to lipids and can tubulate
membranes, potentially facilitating endocytosis93,94.
In summary, actin polymerization by the Arp2/3
complex is regulated by several activator proteins and
numerous proteins that control the activity of the acti-
vators. Unravelling the complex regulation that initiates
and turns off actin polymerization at endocytic sites will
be an important challenge for future research.
Many uses for the actin module
The molecular machinery that is based on actin and the
Arp2/3 complex is well conserved throughout eukaryotes.
Interestingly, the same protein machinery has a central
role in many different cellular processes. Actin and the
Arp2/3 complex are involved in other forms of endo-
cytic internalization including macropinocytosis46,
phagocytosis95 and caveolae-mediated endocytosis96
(FIG. 1). Also, actin and the Arp2/3 complex are central
components in the lamellipodia of motile cells97.
The functional similarities between the actin mesh-
work in lamellipodia and at endocytic sites are intriguing
(FIG. 3b,e). Each lamellipodium is composed of a dense
Arp2/3-complex-nucleated branched actin-filament
meshwork97. Actin filaments are nucleated by the Arp2/3
complex at the leading edge of the lamellipodium, and
actin filaments continuously flow from the leading edge
towards the centre of the cell97. This actin meshwork
is thought to be anchored to the extracellular matrix
through adhesion sites, so that the growing filament ends
can push the leading edge of the cell forward (FIG. 3b).
The proteins that regulate actin polymerization seem
to have analogous localizations in both lamellipodia
and at endocytic sites (FIG. 3b,e). Actin nucleation in
lamellipodia is regulated by Arp2/3-complex activa-
tors, such as WAVE proteins (WASP-family verprolin-
homologous proteins), that localize to the leading
edge of lamellipodia98,99. Actin at endocytic sites seems
to be regulated by WASP-family proteins, namely
N-WASP in mammalian cells, Las17 in S. cerevisiae
and Wsp1 in S. pombe, that are homologous to WAVE.
Mammalian cortactin, which colocalizes with actin
at clathrin-coated pits and endosomes26,45,75, is also
localized throughout the actin-filament meshwork
of lamellipodia100. Furthermore, capping protein
and cofilin, which both regulate actin dynamics
in lamellipodia74, also function at endocytic sites in
S. cerevisiae25,101. The specific functions of these pro-
teins are probably similar in these different processes,
which indicates that they form a functional module
that has retained its basic mechanism of forming a
branched actin-filament meshwork. It also indicates
that this module has been adapted for several different
uses over the course of evolution. The regulation of the
actin–Arp2/3-complex module comes mostly through
the activation or inhibition of the Arp2/3 complex.
The force produced by this module can be harnessed
by proteins (such as endocytic coat proteins or adhesion
proteins) that bind to the actin-filament meshwork.
Although live-cell imaging has provided answers to
many of the questions about endocytosis, it has raised
even more new questions. The endocytic machinery
seems to be much more dynamic than had been previ-
ously appreciated. The regularity with which different
steps follow each other, and the precision with which
numerous proteins assemble at endocytic sites signifies
a remarkable level of molecular choreography. We can
literally see order emerge from the apparent chaos of
the random motions of soluble endocytic proteins as
they assemble to form endocytic structures. How this
ordered sequence of assembly and disassembly takes
place is an important unanswered question. Many loops
of positive and negative feedback are probably involved
in creating the assembly–disassembly sequence.
Other key questions concern the mechanism by
which actin drives endocytosis at the plasma mem-
brane. Is the energy for vesicle budding derived from
actin polymerization, the motor proteins, or both? How
are actin filaments organized at an ultrastructural level
at endocytic sites, and how is their nucleation regulated?
Why is actin more crucial for endocytosis in S. cerevisiae
than in mammalian cells? Does actin have a direct role
in vesicle scission?
New quantitative live-cell imaging and image-analysis
methods will have an important role in answering many
of the remaining questions about the mechanisms of
endocytosis. However, live-cell imaging will be most
powerful when it is used in combination with electron
microscopy and biochemical, pharmacological and
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We thank Y. Sun and V. Okreglak for critically reading the
manuscript. Work in the laboratory of D.G.D. is supported by
grants from the National Institutes of Health.
Competing interests statement
The authors declare no competing financial interests.
The following terms in this article are linked online to:
Entrez Gene: http://www.ncbi.nlm.nih.gov/entrez/query.
BBC1 | SLA1
Abp1 | Ark1 | HIP1R | Las17 | Myo3 | Myo5 | Pan1 | Prk1 | Sla2 | Vrp1
David Drubin’s homepage: http://mcb.berkeley.edu/faculty/
Access to this links box is available online.
414 | JUNE 2006 | VOLUME 7