A transmembrane protein is a type of

integral membrane protein An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All transmembrane proteins can be classified as IMPs, but not all IMPs are transmembrane proteins. IMPs comp ...

that spans the entirety of the

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequently undergo significant conformational changes to move a substance through the membrane. They are usually highly

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

and aggregate and precipitate in water. They require

detergent A detergent is a surfactant or a mixture of surfactants with Cleanliness, cleansing properties when in Concentration, dilute Solution (chemistry), solutions. There are a large variety of detergents. A common family is the alkylbenzene sulfonate ...

s or nonpolar solvents for extraction, although some of them (

beta-barrel In protein structures, a beta barrel (β barrel) is a beta sheet (β sheet) composed of tandem repeats that twists and coils to form a closed toroidal structure in which the first strand is bonded to the last strand (hydrogen bond). Beta-strands ...

s) can be also extracted using denaturing agents. The

peptide sequence Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty ami ...

that spans the membrane, or the transmembrane segment, is largely hydrophobic and can be visualized using the hydropathy plot. Depending on the number of transmembrane segments, transmembrane proteins can be classified as

single-pass membrane protein A single-pass membrane protein also known as single-spanning protein or bitopic protein is a transmembrane protein that spans the lipid bilayer only once. These proteins may constitute up to 50% of all transmembrane proteins, depending on the org ...

s, or as multipass membrane proteins. Some other integral

membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane ...

s are called monotopic, meaning that they are also permanently attached to the membrane, but do not pass through it.

Types

Classification by structure

There are two basic types of transmembrane proteins: alpha-helical and

beta barrel In protein structures, a beta barrel (β barrel) is a beta sheet (β sheet) composed of tandem repeats that twists and coils to form a closed toroidal structure in which the first strand is bonded to the last strand (hydrogen bond). Beta-strands ...

s. Alpha-helical proteins are present in the inner membranes of bacterial cells or the plasma membrane of eukaryotic cells, and sometimes in the

bacterial outer membrane The bacterial outer membrane is found in gram-negative bacteria. Gram-negative bacteria form two lipid bilayers in their cell envelopes - an inner membrane (IM) that encapsulates the cytoplasm, and an outer membrane (OM) that encapsulates the p ...

. This is the major category of transmembrane proteins. In humans, 27% of all proteins have been estimated to be alpha-helical membrane proteins. Beta-barrel proteins are so far found only in outer membranes of

gram-negative bacteria Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...

cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...

s of

gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain ...

, outer membranes of

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

and

chloroplasts A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

, or can be secreted as

pore-forming toxin Pore-forming proteins (PFTs, also known as pore-forming toxins) are usually produced by bacteria, and include a number of protein exotoxins but may also be produced by other organisms such as apple snails that produce perivitellin-2 or earthw ...

s. All beta-barrel transmembrane proteins have simplest up-and-down topology, which may reflect their common evolutionary origin and similar folding mechanism. In addition to the protein domains, there are unusual transmembrane elements formed by peptides. A typical example is

gramicidin A Gramicidin, also called gramicidin D, is a mix of ionophoric antibiotics, gramicidin A, B and C, which make up about 80%, 5%, and 15% of the mix, respectively. Each has 2 isoforms, so the mix has 6 different types of gramicidin molecules. They ...

, a peptide that forms a dimeric transmembrane β-helix. This peptide is secreted by

as an

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...

. A transmembrane polyproline-II helix has not been reported in natural proteins. Nonetheless, this structure was experimentally observed in specifically designed artificial peptides.

Classification by topology

This classification refers to the position of the protein N- and C-termini on the different sides of the

lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cell (biology), cells. The cell membranes of almost all organisms and many viruses a ...

. Types I, II, III and IV are single-pass molecules. Type I transmembrane proteins are anchored to the lipid membrane with a stop-transfer anchor sequence and have their N-terminal domains targeted to the

endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...

(ER) lumen during synthesis (and the extracellular space, if mature forms are located on

cell membranes The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extra ...

). Type II and III are anchored with a signal-anchor sequence, with type II being targeted to the ER lumen with its C-terminal domain, while type III have their N-terminal domains targeted to the ER lumen. Type IV is subdivided into IV-A, with their N-terminal domains targeted to the cytosol and IV-B, with an N-terminal domain targeted to the lumen. The implications for the division in the four types are especially manifest at the time of translocation and ER-bound translation, when the protein has to be passed through the ER membrane in a direction dependent on the type. Group 1 and 2 transmembrane protein

3D structure

Membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane ...

structures can be determined by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

electron microscopy An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing i ...

NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic f ...

. The most common tertiary structures of these proteins are transmembrane helix bundle and

. The portion of the membrane proteins that are attached to the lipid bilayer (see

annular lipid shell Annular lipids (also called shell lipids or boundary lipids) are a set of lipids or lipidic molecules which preferentially bind or stick to the surface of membrane proteins in biological cells. They constitute a layer, or an annulus/ shell, of lip ...

) consist mostly of hydrophobic amino acids.White, Stephen. "General Principle of Membrane Protein Folding and Stability". Stephen White Laboratory Homepage. 10 Nov. 2009. web.

s which have hydrophobic surfaces, are relatively flexible and are expressed at relatively low levels. This creates difficulties in obtaining enough protein and then growing crystals. Hence, despite the significant functional importance of membrane proteins, determining atomic resolution structures for these proteins is more difficult than globular proteins. As of January 2013 less than 0.1% of protein structures determined were membrane proteins despite being 20–30% of the total proteome. Due to this difficulty and the importance of this class of proteins methods of protein structure prediction based on hydropathy plots, the positive inside rule and other methods have been developed.

Thermodynamic stability and folding

Stability of alpha-helical transmembrane proteins

Transmembrane A transmembrane protein is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequently u ...

alpha-helical (α-helical) proteins are unusually stable judging from thermal denaturation studies, because they do not unfold completely within the membranes (the complete unfolding would require breaking down too many α-helical H-bonds in the nonpolar media). On the other hand, these proteins easily ''misfold'', due to non-native aggregation in membranes, transition to the

molten globule In molecular biology, the term molten globule (MG) refers to protein states that are more or less compact (hence the "globule"), but are lacking the specific tight packing of amino acid residues which creates the solid state-like tertiary structu ...

states, formation of non-native

disulfide bonds In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In in ...

, or unfolding of peripheral regions and nonregular loops that are locally less stable. It is also important to properly define the ''

unfolded state In biochemistry, denaturation is a process in which proteins or nucleic acids lose Protein structure, folded structure present in their native state due to various factors, including application of some external stress or compound, such as a st ...

''. The ''unfolded state'' of membrane proteins in detergent micelles is different from that in the thermal denaturation experiments. This state represents a combination of folded hydrophobic α-helices and partially unfolded segments covered by the

. For example, the "unfolded"

bacteriorhodopsin Bacteriorhodopsin (Bop) is a protein used by Archaea, most notably by Haloarchaea, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The res ...

in SDS micelles has four transmembrane α-helices folded, while the rest of the protein is situated at the micelle-water interface and can adopt different types of non-native

amphiphilic In chemistry, an amphiphile (), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'', nonpolar) properties. Such a compound is called amphiphilic or amphipathic. Amphiphilic c ...

structures. Free energy differences between such detergent-denatured and native states are similar to stabilities of water-soluble proteins (< 10 kcal/mol).

Folding of α-helical transmembrane proteins

Refolding of α-helical transmembrane proteins ''in vitro'' is technically difficult. There are relatively few examples of the successful refolding experiments, as for

. ''In vivo'', all such proteins are normally folded co-translationally within the large transmembrane

translocon The translocon (also known as a translocator or translocation channel) is a complex of proteins associated with the translocation of polypeptides across membranes. In eukaryotes the term translocon most commonly refers to the complex that transpor ...

. The translocon channel provides a highly heterogeneous environment for the nascent transmembrane α-helices. A relatively polar amphiphilic α-helix can adopt a transmembrane orientation in the translocon (although it would be at the membrane surface or unfolded ''in vitro''), because its polar residues can face the central water-filled channel of the translocon. Such mechanism is necessary for incorporation of polar α-helices into structures of transmembrane proteins. The amphiphilic helices remain attached to the translocon until the protein is completely synthesized and folded. If the protein remains unfolded and attached to the translocon for too long, it is degraded by specific "quality control" cellular systems.

Stability and folding of beta-barrel transmembrane proteins

Stability of beta barrel (β-barrel) transmembrane proteins is similar to stability of water-soluble proteins, based on chemical denaturation studies. Some of them are very stable even in chaotropic agents and high temperature. Their folding ''in vivo'' is facilitated by water-soluble chaperones, such as protein Skp. It is thought that β-barrel membrane proteins come from one ancestor even having different number of sheets which could be added or doubled during evolution. Some studies show a huge sequence conservation among different organisms and also conserved amino acids which hold the structure and help with folding.

3D structures

Light absorption-driven transporters

Bacteriorhodopsin Bacteriorhodopsin (Bop) is a protein used by Archaea, most notably by Haloarchaea, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The res ...

-like proteins including

rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...

(see also

opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...

) * Bacterial

photosynthetic reaction centre A photosynthetic reaction center is a complex of several proteins, biological pigments, and other co-factors that together execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from ...

s and

photosystem Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems ...

s I and II *

Light-harvesting complex In biology, a light-harvesting complex or LHC is an aggregate consisting of proteins bound with chromophores (chlorophylls and carotenoids) that play a key role in photosynthesis. LHCs are arrayed around photosynthetic reaction centers in both pl ...

es from

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and

Oxidoreduction-driven transporters

* Transmembrane cytochrome b-like proteins: coenzyme Q - cytochrome c reductase (cytochrome bc1 );

cytochrome b6f complex The cytochrome ''b''6''f'' complex (plastoquinol/plastocyanin reductase or plastoquinol/plastocyanin oxidoreductase; ) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, that catalyzes the ...

; formate dehydrogenase, respiratory

nitrate reductase Nitrate reductases are molybdoenzymes that reduce nitrate () to nitrite (). This reaction is critical for the production of protein in most crop plants, as nitrate is the predominant source of nitrogen in fertilized soils. Types Euka ...

; succinate - coenzyme Q reductase (fumarate reductase); and

succinate dehydrogenase Succinate dehydrogenase (SDH) or succinate-coenzyme Q reductase (SQR) or respiratory complex II is an enzyme complex, found in many bacterial cells and in the inner mitochondrial membrane of eukaryotes. It is the only enzyme that participates ...

. See

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

. *

Cytochrome c oxidase The enzyme cytochrome c oxidase or Complex IV (was , now reclassified as a translocasEC 7.1.1.9 is a large transmembrane protein complex found in bacteria, archaea, and the mitochondria of eukaryotes. It is the last enzyme in the Cellular respir ...

s from

and

Electrochemical potential-driven transporters

* Proton or sodium translocating F-type and V-type

ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, ATP hydrolase, adenosine triphosphatase) are a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion or ...

P-P-bond hydrolysis-driven transporters

* P-type calcium ATPase (five different conformations) * Calcium ATPase regulators phospholamban and sarcolipin * ABC transporters * General

secretory pathway Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell (biology), cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. Th ...

(Sec)

(preprotein translocase SecY)

Porters (uniporters, symporters, antiporters)

Mitochondrial A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

carrier protein A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembrane proteins; that is they ...

s * Major Facilitator Superfamily (Glycerol-3-phosphate transporter, Lactose

permease The permeases are membrane transport proteins, a class of multipass transmembrane proteins that allow the diffusion of a specific molecule in or out of the cell in the direction of a concentration gradient, a form of facilitated diffusion Facil ...

, and Multidrug transporter EmrD) * Resistance-nodulation-cell division (multidrug efflux transporter AcrB, see

multidrug resistance Multiple drug resistance (MDR), multidrug resistance or multiresistance is antimicrobial resistance shown by a species of microorganism to at least one antimicrobial drug in three or more antimicrobial categories. Antimicrobial categories are ...

) * Dicarboxylate/amino acid:cation symporter (proton glutamate symporter) * Monovalent cation/proton antiporter (Sodium/proton antiporter 1 NhaA) *

Neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

sodium symporter * Ammonia transporters * Drug/Metabolite Transporter (small multidrug resistance transporter EmrE - the structures are retracted as erroneous)

Alpha-helical channels including ion channels

Voltage-gated ion channel Voltage-gated ion channels are a class of transmembrane proteins that form ion channels that are activated by changes in a Cell (biology), cell's electrical membrane potential near the channel. The membrane potential alters the conformation of t ...

like, including

potassium channel Potassium channels are the most widely distributed type of ion channel found in virtually all organisms. They form potassium-selective pores that span cell membranes. Potassium channels are found in most cell types and control a wide variety of ...

s KcsA and KvAP, and

inward-rectifier potassium ion channel Inward-rectifier potassium channels (Kir, IRK) are a specific Lipid-gated_ion_channels, lipid-gated subset of potassium channels. To date, seven subfamilies have been identified in various mammalian cell types, plants, and bacteria. They are acti ...

Kirbac * Large-conductance mechanosensitive channel, MscL * Small-conductance mechanosensitive ion channel (MscS) * CorA metal ion transporters *

Ligand-gated ion channel Ligand-gated ion channels (LICs, LGIC), also commonly referred to as ionotropic receptors, are a group of transmembrane ion-channel proteins which open to allow ions such as sodium, Na+, potassium, K+, calcium, Ca2+, and/or chloride, Cl− to ...

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

receptors (

acetylcholine receptor An acetylcholine receptor (abbreviated AChR) or a cholinergic receptor is an integral membrane protein that responds to the binding of acetylcholine, a neurotransmitter. Classification Like other transmembrane receptors, acetylcholine receptor ...

) *

Aquaporin Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of ...

s *

Chloride channel Chloride channels are a superfamily of poorly understood ion channels specific for chloride. These channels may conduct many different ions, but are named for chloride because its concentration ''in vivo'' is much higher than other anions. Several ...

s * Outer membrane auxiliary proteins (polysaccharide transporter) - α-helical transmembrane proteins from the outer bacterial membrane

Enzymes

Methane monooxygenase Methane monooxygenase (MMO) is an enzyme capable of oxidizing the C-H bond in methane as well as other alkanes. Methane monooxygenase belongs to the class of oxidoreductase enzymes (). There are two forms of MMO: the well-studied soluble form (s ...

Rhomboid protease The rhomboid proteases are a family of enzymes that exist in almost all species. They are proteases: they cut the polypeptide chain of other proteins. This proteolytic cleavage is irreversible in cell (biology), cells, and an important type of cel ...

Disulfide bond In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inor ...

formation protein (DsbA-DsbB complex)

Proteins with single transmembrane alpha-helices

* Subunits of

T cell receptor The T-cell receptor (TCR) is a protein complex, located on the surface of T cells (also called T lymphocytes). They are responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules. ...

complex * Cytochrome c

nitrite reductase Nitrite reductase refers to any of several classes of enzymes that catalyze the reduction of nitrite. There are two classes of NIR's. A multi haem enzyme reduces NO2− to a variety of products. Copper containing enzymes carry out a single el ...

complex * Glycophorin A dimer * Inovirus ( filamentous phage) major coat protein * Pilin *

Pulmonary surfactant Pulmonary surfactant is a surface-active complex of phospholipids and proteins formed by Type II cells, type II alveolar cells. The proteins and lipids that make up the surfactant have both hydrophilic and hydrophobic regions. By adsorption, adso ...

-associated protein *

Monoamine oxidase Monoamine oxidases (MAO) () are a family of enzymes that catalyze the oxidation of monoamines, employing oxygen to clip off their amine group. They are found bound to the outer membrane of mitochondria in most cell types of the body. The fi ...

s A and B *

Fatty acid amide hydrolase Fatty-acid amide hydrolase 1 (FAAH) is a member of the serine hydrolase family of enzymes. It was first shown to break down anandamide (AEA), an ''N''-acylethanolamine (NAE) in 1993. In humans, it is encoded by the gene ''FAAH''.; Function ...

Cytochrome P450 oxidase Cytochromes P450 (P450s or CYPs) are a superfamily of enzymes containing heme as a cofactor that mostly, but not exclusively, function as monooxygenases. However, they are not omnipresent; for example, they have not been found in ''Escherich ...

s * Corticosteroid 11β-dehydrogenases . * Signal Peptide Peptidase

Beta-barrels composed of a single polypeptide chain

* Beta barrels from eight beta-strands and with "shear number" of ten (''n=8, S=10''). They include: ** OmpA-like transmembrane domain (OmpA) ** Virulence-related outer membrane protein family (OmpX) ** Outer membrane protein W family (OmpW) ** Antimicrobial peptide resistance and lipid A acylation protein family (PagP) ** Lipid A deacylase PagL ** Opacity family porins (NspA) * Autotransporter domain (''n=12,S=14'') * FadL outer membrane protein transport family, including

Fatty acid In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...

transporter FadL (''n=14,S=14'') * General bacterial porin family, known as trimeric porins (''n=16,S=20'') * Maltoporin, or sugar porins (''n=18,S=22'') * Nucleoside-specific porin (''n=12,S=16'') * Outer membrane phospholipase A1(''n=12,S=16'') * TonB-dependent receptors and their plug domain. They are ligand-gated outer membrane channels (''n=22,S=24''), including

cobalamin Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in metabolism. One of eight B vitamins, it serves as a vital cofactor (biochemistry), cofactor in DNA synthesis and both fatty acid metabolism, fatty acid and amino a ...

transporter BtuB, Fe(III)-pyochelin receptor FptA, receptor FepA, ferric hydroxamate uptake receptor FhuA, transporter FecA, and pyoverdine receptor FpvA * Outer membrane protein OpcA family (''n=10,S=12'') that includes outer membrane

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...

OmpT and adhesin/invasin OpcA protein * Outer membrane protein G porin family (''n=14,S=16'') Note: ''n'' and ''S'' are, respectively, the number of beta-strands and the "shear number" of the

Beta-barrels composed of several polypeptide chains

* Trimeric autotransporter (''n=12,S=12'') * Outer membrane efflux proteins, also known as trimeric outer membrane factors (n=12,S=18) including TolC and multidrug resistance proteins * MspA porin (octamer, ''n=S=16'') and α-hemolysin (heptamer ''n=S=14'') . These proteins are secreted.

References

{{DEFAULTSORT:Transmembrane Protein Integral membrane proteins