R. Michael Garavito's research while affiliated with Michigan State University and other places
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Publications (133)
The ancient protein TSPO (translocator protein 18kD) is found in all kingdoms and was originally identified as a binding site of benzodiazepine drugs. Its physiological function remains unclear, although porphyrins are conserved ligands. Several crystal structures of bacterial TSPO and nuclear magnetic resonance structures of a mouse form have reve...
The translocator protein 18 kDa (TSPO) was previously known as the peripheral benzodiazepine receptor (PBR) in eukaryotes, where it is mainly localized to the mitochondrial outer membrane. Considerable evidence indicates that it plays regulatory roles in steroidogenesis and apoptosis, as well as being involved in various human diseases, such as met...
The 18-kilodalton translocator protein (TSPO), proposed to be a key player in cholesterol transport into mitochondria, is highly expressed in steroidogenic tissues, metastatic cancer, and inflammatory and neurological diseases such as Alzheimer's and Parkinson's. TSPO ligands, including benzodiazepine drugs, are implicated in regulating apoptosis a...
Translocator protein 18kDa is a mitochondrial outer membrane protein that was first recognized in mammalian systems as the peripheral benzodiazepine receptor (PBR) and in Rhodobacter sphaeroides as the tryptophan-rich sensory protein (TspO). Although many aspects of its function in bacteria and mitochondria remain unclear and controversial, TSPO is...
Beta-hydroxyacid dehydrogenase (β-HAD) genes have been identified in all sequenced genomes of eukaryotes and prokaryotes. Their gene products catalyze the NAD(+)- or NADP(+)-dependent oxidation of various β-hydroxy acid substrates into their corresponding semialdehyde. In many fungal and bacterial genomes, multiple β-HAD genes are observed leading...
Oxicams are widely used non-steroidal anti-inflammatory drugs (NSAIDs), but little is known about the molecular basis of the interaction with their target enzymes, the cyclooxygenases (COX). Isoxicam is a non-selective inhibitor of COX-1 and COX-2 whereas meloxicam displays some selectivity for COX-2. Here we report crystal complexes of COX-2 with...
Hot, Toxic Eukaryote
Unusually, the single-celled eukaryote red alga, Galdieria sulphuraria , can thrive in hot, acidic springs. This organism is endowed with extraordinary metabolic talents and can consume a variety of strange carbohydrates, as well as turn on photosynthesis when the food runs out. Schönknecht et al. (p. 1207 ; see the Perspective...
Electrochemical impedance spectroscopy (EIS) was used to characterize voltage-dependent closure of PorB class II (PorBII) porin from Neisseria meningitidis incorporated in a tethered bilayer lipid membrane (tBLM). The tBLM's lower leaflet was fabricated by depositing a self assembled monolayer (SAM) of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethano...
Sequential adsorption of poly(styrene sulfonate) (PSS) and proteases in porous nylon yields enzymatic membrane reactors for limited protein digestion. Although a high local enzyme density (∼30 mg/cm(3)) and small pore diameters in the membrane lead to digestion in <1 s, the low membrane thickness (170 μm) affords control over residence times at the...
Succinic semialdehyde reductase (SSAR) is an important enzyme involved in γ-aminobutyrate (GABA) metabolism. By converting succinic semialdehyde (SSA) to γ-hydroxybutyrate (GHB), the SSAR facilitates an alternative pathway for GABA degradation. In this study, we identified SSARs from Geobacter sulfurreducens and Geobacter metallireducens (GsSSAR an...
Sucrose transport is the central system for the allocation of carbon resources in vascular plants. During growth and development, plants control carbon distribution by coordinating sites of sucrose synthesis and cleavage in different plant organs and different cellular locations. Sucrose synthase, which reversibly catalyzes sucrose synthesis and cl...
Human dynamin-like protein 1 (DLP-1) is involved in the fission of mitochondrial outer membranes, a process that helps to
maintain mitochondrial morphology and to reduce the accumulation of functional and structural defects in mitochondria. DLP-1
is a ∼80 kDa membrane-interacting protein and contains a GTPase domain, a middle domain, a putative PH-...
Intestinal tumors in Apc(Min/+) mice are suppressed by over-production of HPGDS, which is a glutathione transferase that forms prostaglandin D(2) (PGD(2)). We characterized naturally occurring HPGDS isoenzymes, to see if HPGDS variation is associated with human colorectal cancer risk. We used DNA heteroduplex analysis and sequencing to identify HPG...
The (pro)renin receptor (PRR) is an important component of the renin-angiotensin system (RAS), which regulates blood pressure and cardiovascular function. The integral membrane protein PRR contains a large extracellular domain (∼310 amino acids), a single transmembrane domain (∼20 amino acids) and an intracellular domain (∼19 amino acids). Although...
GDP-D-mannose 4,6-dehydratase catalyzes the first step in the de novo synthesis of GDP-L-fucose, the activated form of L-fucose, which is a component of glycoconjugates in plants known to be important to the development and strength of stem tissues. We have determined the three-dimensional structure of the MUR1 dehydratase isoform from Arabidopsis...
Kinetic studies and analysis of the products formed by native and mutant forms of ovine prostaglandin endoperoxide H synthase-1 (oPGHS-1) have suggested that arachidonic acid (AA) can exist in the cyclooxygenase active site of the enzyme in three different, catalytically competent conformations that lead to prostaglandin Gâ (PGGâ), 11Rhydroperoxyei...
Regulated intramembrane proteolysis (RIP) involves cleavage of a transmembrane segment of a protein. RIP governs diverse processes in a wide variety of organisms and is carried out by different types of intramembrane proteases (IPs), including a large family of metalloproteases. The Bacillus subtilis SpoIVFB protein is a putative metalloprotease th...
The rare 6-deoxysugar D-rhamnose is a component of bacterial cell surface glycans, including the D-rhamnose homopolymer produced by Pseudomonas aeruginosa, called A-band O polysaccharide. GDP-D-rhamnose synthesis from GDP-D-mannose is catalyzed by two enzymes. The first is a GDP-D-mannose-4,6-dehydratase (GMD). The second enzyme, RMD, reduces the G...
To test the hypothesis that activation of the transient receptor potential vanilloid 4 (TRPV4) channel conveys a hypotensive effect that is enhanced during salt load, male Wistar rats fed a normal-sodium (0.5%) or high-sodium (HS; 4%) diet for 3 weeks were given 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), a specific TRPV4 activator, in the pre...
PorB class II from Neisseria meningitidis is a pore-forming, outer-membrane protein that can translocate to the host-cell membrane during Neisserial infections. This report describes development of tethered bilayer lipid membrane (tBLM) system to measure PorB conductance properties. The tBLM was fabricated by depositing a self-assembled monolayer o...
A tethered bilayer lipid membrane (tBLM) was fabricated on a gold electrode using 1,2-dipalmitoyl-sn-glycero-phosphothioethanol as a tethering lipid and the membrane fractions of Saccharomyces pombe yeast cells to deposit the upper leaflet. The membrane fractions were characterized using transmission electron microscopy and dynamic light scattering...
X-ray diffraction analysis at 1.5 A resolution has confirmed the helical conformation of a de novo designed 18-residue peptide. However, the crystal structure reveals the formation of continuous molecular layers of parallel-packed amphiphilic helices as a result of much more extensive helix-helix interactions than predicted. The crystal packing arr...
The plant sulfolipid sulfoquinovosyldiacylglycerol accounts for a large fraction of organic sulfur in the biosphere. Aside
from sulfur amino acids, sulfolipid represents a considerable sink for sulfate in plants. Plant sulfolipid is found in the
photosynthetic membranes of plastids and provides negative charge in the thylakoid membrane where it is...
The modification of the nonselective nonsteroidal anti-inflammatory drug, indomethacin, by amidation presents a promising strategy for designing novel cyclooxygenase (COX)-2-selective inhibitors. A series of alpha-substituted indomethacin ethanolamides, which exist as R/S-enantiomeric pairs, provides a means to study the impact of stereochemistry o...
Dietary fish oil containing omega 3 highly unsaturated fatty acids has cardioprotective and anti-inflammatory effects. Prostaglandins (PGs) and thromboxanes are produced in vivo both from the omega 6 fatty acid arachidonic acid (AA) and the omega 3 fatty acid eicosapentaenoic acid (EPA). Certain beneficial effects of fish oil may result from altere...
Specific interactions between lipids and membrane proteins have been observed in recent high-resolution crystal structures of membrane proteins. A number of cytochrome oxidase structures were analyzed, along with many amino acid sequences of membrane-spanning regions aligned according to their location in the membrane. The results reveal conservati...
Cytochrome c oxidase (CcO) transfers protons from the inner surface of the enzyme to the buried O2 reduction site through two different pathways, termed K and D, and from the outer surface via an undefined route. These proton paths can be inhibited by metals such as zinc or cadmium, but the sites of inhibition have not been established. Anomalous d...
IntroductionThrough-Space versus Through-Bond Interactions in Photoinduced Electron Transfer of Acyclic BichromophoresDiscussionConclusions
Well ordered reproducible crystals of cytochrome c oxidase (CcO) from Rhodobacter sphaeroides yield a previously unreported structure at 2.0 Å resolution that contains the two catalytic subunits and a number of alkyl chains of lipids and detergents. Comparison with crystal structures of other bacterial and mammalian CcOs reveals that the positions...
The flavoenzyme acyl-CoA oxidase (ACX) catalyzes the first committed step in beta-oxidation and is required for the biosynthesis of jasmonic acid, a signaling molecule involved in plant defense. Recently, a mutant in tomato was identified that is deficient in jasmonic acid production and compromised in its wound response. This results from a single...
The prostaglandin endoperoxide H2 synthase (PGHS) isozymes 1 and 2 are membrane bound, heme-dependent enzymes that catalyze the committed step in the conversion of polyunsaturated fatty acids to prostanoids and thromboxanes. The PGHS isozymes, which are also known as cyclooxygenases, produce prostaglandin H2 in two sequential enzymatic steps – a bi...
UDP-sulfoquinovose synthase, SQD1, catalyzes the transfer of sulfite to UDP-glucose giving rise to UDP-sulfoquinovose, which is the head group donor for the biosynthesis of the plant sulfolipid sulfoquinovosyldiacylglyerol. The native SQD1 enzyme of spinach exists as a 250 kDa heteroprotein complex with much higher affinity for the substrate sulfit...
Unicellular algae serve as models for the study and discovery of metabolic pathways, for the functional dissection of cell biological processes such as organellar division and cell motility, and for the identification of novel genes and gene functions. The recent completion of several algal genome sequences and expressed sequence tag collections an...
Kinetic studies and analysis of the products formed by native and mutant forms of ovine prostaglandin endoperoxide H synthase-1 (oPGHS-1) have suggested that arachidonic acid (AA) can exist in the cyclooxygenase active site of the enzyme in three different, catalytically competent conformations that lead to prostaglandin G2 (PGG2), 11R-hydroperoxye...
When we think of extremophiles, organisms adapted to extreme environments, prokaryotes come to mind first. However, the unicellular red micro-alga Galdieria sulphuraria (Cyanidiales) is a eukaryote that can represent up to 90% of the biomass in extreme habitats such as hot sulfur springs with pH values of 0-4 and temperatures of up to 56 degrees C....
The TDP-vancosaminyltransferase GtfD catalyzes the attachment of L-vancosamine to a monoglucosylated heptapeptide intermediate during the final stage of vancomycin biosynthesis. Glycosyltransferases from this and similar antibiotic pathways are potential tools for the design of new compounds that are effective against vancomycin resistant bacterial...
d-Rhamnose is a rare 6-deoxy monosaccharide primarily found in the lipopolysaccharide of pathogenic bacteria, where it is involved in host-bacterium interactions and the establishment of infection. The biosynthesis of d-rhamnose proceeds through the conversion of GDP-d-mannose by GDP-d-mannose 4,6-dehydratase (GMD) to GDP-4-keto-6-deoxymannose, whi...
During the biosynthesis of the vancomycin-class antibiotic chloroeremomycin, TDP-epi-vancosaminyltransferase GtfA catalyzes the attachment of 4-epi-vancosamine from a TDP donor to the beta-OHTyr-6 of the aglycone cosubstrate. Glycosyltransferases from this pathway are potential tools for the combinatorial design of new antibiotics that are effectiv...
Cyclooxygenases-1 and -2 (COX-1 and COX-2, also known as prostaglandin H2 synthases-1 and -2) catalyze the committed step in prostaglandin synthesis. COX-1 and -2 are of particular interest because they are the major targets of nonsteroidal antiinflammatory drugs (NSAIDs) including aspirin, ibuprofen, and the new COX-2-selective inhibitors. Inhibit...
GDP-D-mannose 4,6-dehydratase catalyzes the first step in the de novo synthesis of GDP-L-fucose, the activated form of L-fucose, which is a component of glycoconjugates in plants known to be important to the development and strength of stem tissues. We have determined the three-dimensional structure of the MUR1 dehydratase isoform from Arabidopsis...
Prostaglandin H synthase 2 (also known as cyclooxygenase-2) is thought to play a role in the prevention of colon cancer by aspirin, an inhibitor of the enzyme. We used DNA heteroduplex analysis to screen the prostaglandin H synthase 2 gene, to search for naturally occurring enzyme variants that may simulate the effects of aspirin. We found among Af...
Prostaglandin endoperoxide H synthase-1 and -2 (PGHS-1 and -2) convert arachidonic acid (AA) to prostaglandin H2 (PGH2) in the committed step in prostaglandin biosynthesis. Although the cyclooxygenase activity favors AA as the substrate, both isoforms will oxygenate a variety of 18–22 carbon fatty acids with reduced efficiencies. In this review, we...
Despite the marked differences in their physiological roles, the structures and catalytic functions of the prostaglandin H2 endoperoxide synthases-1 and -2 (PGHS-1 and -2) are almost completely identical. These integral membrane proteins catalyze the conversion of arachidonic acid to PGG2 and finally to PGH2. The crystal structures of PGHS-1 and -2...
Prostaglandin endoperoxide H synthases-1 and -2 (PGHSs) can oxygenate 18-22 carbon polyunsaturated fatty acids, albeit with varying efficiencies. Here we report the crystal structures of eicosapentaenoic acid (EPA, 20:5 n-3) and linoleic acid (LA, 18:2 n-6) bound in the cyclooxygenase active site of Co(3+) protoporphyrin IX-reconstituted ovine PGHS...
Detergents are invaluable tools for studying membrane proteins. However, these deceptively simple, amphipathic molecules exhibit complex behavior when they self-associate and interact with other molecules. The phase behavior and assembled structures of detergents are markedly influenced not only by their unique chemical and physical properties but...
Members of the vancomycin group of glycopeptide antibiotics have an oxidatively crosslinked heptapeptide scaffold decorated at the hydroxyl groups of 4-OH-Phegly4 or beta-OH-Tyr6 with mono- (residue 6) or disaccharides (residue 4). The disaccharide in vancomycin itself is L-vancosamine-1,2-glucose, and in chloroeremomycin it is L-4-epi-vancosamine-...
Prostaglandin endoperoxide H synthases (PGHSs) catalyze the committed step in the biosynthesis of prostaglandins and thromboxane,
the conversion of arachidonic acid, two molecules of O2, and two electrons to prostaglandin endoperoxide H2 (PGH2). Formation of PGH2 involves an initial oxygenation of arachidonate to yield PGG2 catalyzed by the cycloox...
Prostaglandin endoperoxide H synthases-1 and -2 (PGHSs) catalyze the committed step in prostaglandin biosynthesis. Both isozymes can oxygenate a variety of related polyunsaturated fatty acids. We report here the x-ray crystal structure of dihomo-γ-linolenic acid (DHLA) in the cyclooxygenase site of PGHS-1 and the effects of active site substitution...
Prostaglandin endoperoxide H synthases (PGHSs) catalyze the committed step in the biosynthesis of prostaglandins and thromboxane,
the conversion of arachidonic acid, two molecules of O2, and two electrons to prostaglandin endoperoxide H2 (PGH2). Formation of PGH2 involves an initial oxygenation of arachidonate to yield PGG2 catalyzed by the cycloox...
The sulfolipid sulfoquinovosyldiacylglycerol is a component of plant photosynthetic membranes and represents one of the few naturally occurring sulfonic acids with detergent properties. Sulfolipid biosynthesis involves the transfer of sulfoquinovose, a 6-deoxy-6-sulfoglucose, from UDP-sulfoquinovose to diacylglycerol. The formation of the sulfonic...
Three-dimensional microcrystals of OmpC osmoporin were air-dried slowly and imaged in air with an atomic force microscope (AFM). The overall structural features in AFM images are in good agreement with the X-ray diffraction data of these OmpC osmoporin crystals: monoclinic P21 with the unit cell constants a=117.6 Å, b=110 Å, c=298.4 Å, β=97°. Such...
Prostaglandin endoperoxide H synthases-1 and -2 (PGHS-1 and -2) catalyze the conversion of arachidonic acid, two molecules of O2 and two electrons to PGH2. This is the committed step in the formation of prostaglandins and thromboxane A2 [1]. Crystallographic studies of enzyme inhibitor complexes have suggested that the cyclooxygenase active sites o...
We have investigated the cleavage of the bond between the heme iron and the proximal heme ligand, His388, in ferrous ovine prostaglandin endoperoxide H synthase-1 (oPGHS-1) on nitric oxide (NO) binding by using resonance Raman spectroscopy. The Fe−NO and N−O vibrations are observed at 526 and 1667 cm-1, respectively, which is indicative of a five-c...
Prostaglandin H synthase-1 and -2 (PGHS-1 and -2) catalyze the committed step in prostaglandin synthesis and are targets for nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin. We have determined the structure of PGHS-1 at 3 angstrom resolution with arachidonic acid (AA) bound in a chemically productive conformation. The fatty acid adopts a...
We have developed a protocol to purify apo-ovine (o) prostaglandin endoperoxide H(2) synthase-1 (PGHS-1) to homogeneity from ram seminal vesicles. The resulting apo enzyme can then be reconstituted with Co(3+)-protoporphyrin IX instead of Fe(3+)-protoporphyrin IX to produce a native-like, but functionally inert, enzyme suitable for the production o...
Prostaglandin endoperoxide H synthases-1 and -2 (PGHS-1 and -2) convert arachidonic acid to prostaglandin H-2 (PGH(2)), the committed step in prostaglandin and thromboxane formation. Interaction of peroxides with the heme sites in PGHSs generates a tyrosyl radical that catalyzes subsequent cyclooxygenase chemistry. To study the peroxidase reaction...
The prostaglandin endoperoxide H synthases-1 and 2 (PGHS-1 and PGHS-2; also cyclooxygenases-1 and 2, COX-1 and COX-2) catalyze the committed step in prostaglandin synthesis. PGHS-1 and 2 are of particular interest because they are the major targets of nonsteroidal anti-inflammatory drugs (NSAIDs) including aspirin, ibuprofen, and the new COX-2 inhi...
Prostaglandin endoperoxide H synthases 1 and -2 (PGHS-1 and -2) convert arachidonic acid and O2 (along with two reducing equivalents) to PGH2 — the committing step in the formation of prostanoids (Smith and DeWitt 1996; Smith et al. 1996). PGHS-1 is often referred to as the constitutive enzyme, whereas PGHS-2 is known as the inducible isoform. They...
Prostaglandin endoperoxide H synthases 1 and 2 (PGHS-1 and -2) are the major targets of nonsteroidal anti-inflammatory drugs. Both isozymes are integral membrane proteins but lack transmembrane domains. X-ray crystallographic studies have led to the hypothesis that PGHS-1 and -2 associate with only one face of the membrane bilayer through a novel,...
Despite the marked differences in their physiological roles, the structures and catalytic functions of the cyclooxygenase isozymes COX-1 and -2 are virtually identical. Nevertheless, a handful of amino acid substitutions give rise to subtle differences in ligand binding between the two isoforms. These 'small' alterations of isozyme structure are su...
The SQD1 enzyme is believed to be involved in the biosynthesis of the sulfoquinovosyl headgroup of plant sulfolipids, catalyzing the transfer of SO3− to UDP-glucose. We have determined the structure of the complex of SQD1 from Arabidopsis thaliana with NAD+ and the putative substrate UDP-glucose at 1.6-Å resolution. Both bound ligands are completel...
Arg-120 is located near the mouth of the hydrophobic channel that forms the cyclooxygenase active site of prostaglandin endoperoxide H synthases (PGHSs)-1 and -2. Replacement of Arg-120 of ovine PGHS-1 with a glutamine increases the apparent Km of PGHS-1 for arachidonate by 1,000-fold (Bhattacharyya, D. K., Lecomte, M., Rieke, C. J., Garavito, R. M...
The Type I isozyme of mammalian hexokinase has evolved by a gene duplication-fusion mechanism, with resulting internal duplication of sequence and ligand binding sites. However, 1:1 binding stoichiometry indicates that only one of these is available for binding the product inhibitor, Glc-6-P; the location of that site, in the N- or C-terminal half,...
The structure determinations of the cytochrome bc1 complex and the prokaryotic potassium channel demonstrate that a wider range of membrane proteins are now amenable to study by X-ray crystallography. Furthermore, the structures of porins and interfacial membrane proteins show that membrane structural biology is becoming a mature and productive fie...
We have determined the structures of the glucose-6-phosphate (G6P)-inhibitable 100,000 Mr Type I hexokinase from rat and the G6P-sensitive 50,000 Mr hexokinase from Schistosoma mansoni at a resolution of 2.8 and 2.6 A respectively. The structures define the glucose and G6P binding sites in these enzymes, suggest the mechanisms of intradomain G6P in...
Prostaglandin H synthase (PGHS) is the first enzyme in the prostaglandin pathway of the arachidonate cascade (1). It is a bifunctional enzyme that catalyzes the conversion of arachidonic acid into prostaglandin H2 (PGH2). Two isozymes of the enzyme (PGHS-1 and PGHS-2) exist and have different modes of expression and tissue distributions (2). The fi...
The crystal structure of human cyclooxygenase-2 offers a unique opportunity for the design of new nonsteroidal anti-inflammatory drugs and may also provide insights into the mechanisms of membrane protein targeting and the role of prostaglandin physiology in inflammation and disease.
The cyclooxygenase activity of the membrane protein prostaglandin H2 synthase isoform 1 (PGHS-1) is the target of the nonsteroidal antiinflammatory drugs (NSAIDs). The X-ray crystal structures of PGHS-1 in complex with the NSAIDs flurbiprofen and bromoaspirin have been determined previously [Picot, D., et al. (1994) Nature 367, 243-249; Loll, P. J....
The crystal structure of 1-(4-iodobenzoyl)-5-methoxy-2-methyl-3-indoleacetic acid, C19H16INO4, an analog of indomethacin, is reported. Bond distances and angles in the title compound closely resemble those reported for indomethacin and reflect the presence of steric strain at the site of the linkage between the 4-iodobenzoyl group and the indole mo...
Crystallizing membrane proteins remains a challenging endeavor despite the increasing number of membrane protein structures solved by X-ray crystallography. The critical factors in determining the success of the crystallization experiments are the purification and preparation of membrane protein samples. Moreover, there is the added complication th...
The crystal structure of 2-bromoacetoxybenzoic acid, C9H7BrO4, shows it to be a close structural analog of aspirin. The carboxylic acid moiety is twisted by 7.7 (4) degrees out of the plane of the aromatic ring. The acetyl group, like that of aspirin, shows bond-angle distortions from ideal values while remaining essentially planar. The Br atom is...
There are two isozymes of prostaglandin endoperoxide H synthase (cyclooxygenase) called PGHS-1 and -2 (or COX-1 and -2). Both isozymes catalyze the conversion of arachidonate to PGH2, the committed step in the formation of prostaglandins and thromboxane. PGHS-1 and PGHS-2 are encoded by different genes. PGHS-1 is expressed constitutively in most ti...
Prostaglandin H2 synthase (PGHS; EC 1.14.99.1; cyclooxygenase or COX) catalyses the two-step conversion of arachidonic acid to prostaglandin G2 and thence to prostaglandin H2, the first committed steps in the biosynthesis of prostaglandins1,2. While COX catalyses both steps of this catalytic process (known as the cyclooxygenase and peroxidase steps...
OmpF porin is a trimeric integral membrane protein responsible for the passive transport of small hydrophilic molecules, such as nutrients and waste products, across the outer membrane of Escherichia coli. Very few membrane proteins have been crystallized in three dimensions, yet this stable protein can be obtained in several crystal forms. Compari...
The high-resolution structures of five porins have been solved by X-ray crystallography including the trigonal crystal form of the trimeric OmpF porin from Escherichia coli. In an accompanying article, the structure of the tetragonal form of OmpF porin is presented. In contrast to the trigonal crystal form, the protein surfaces normally in contact...