Membrane proteins are proteins that interact with biological membranes. They are one of the common types of protein along with soluble globular proteins, fibrous proteins, and disordered proteins. They are targets of over 50% of all modern medicinal drugs. It is estimated that 20–30% of all genes in most genomes encode membrane proteins.
Membrane proteins perform a variety of functions vital to the survival of organisms:
- Membrane receptor proteins relay signals between the cell's internal and external environments.
- Transport proteins move molecules and ions across the membrane. They can be categorized according to the Transporter Classification database.
- Membrane enzymes may have many activities, such as oxidoreductase, transferase or hydrolase.
- Cell adhesion molecules allow cells to identify each other and interact. For example proteins involved in immune response.
A slightly different classification is to divide all membrane proteins to integral and amphitropic. Amphitropic proteins exist in two alternative states: a water-soluble and a lipid bilayer-bound. The amphitropic protein category includes water-soluble channel-forming polypeptide toxins, which associate irreversibly with membranes, but excludes peripheral proteins that interact with other membrane proteins rather than with lipid bilayer.
Integral membrane proteins
Integral membrane proteins are permanently attached to the membrane. Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents. They can be classified according to their relationship with the bilayer:
- Integral polytopic proteins, also known as "transmembrane proteins," are integral membrane proteins that span across the membrane at least once. They have one of two tertiary structures:
- Integral monotopic proteins are integral membrane proteins that are attached to only one side of the membrane and do not span the whole way across.
Peripheral membrane proteins
Peripheral membrane proteins are temporarily attached either to the lipid bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and other non-covalent interactions. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations.
Polypeptide toxins and many antibacterial peptides, such as colicins or hemolysins, and certain proteins involved in apoptosis, are sometimes considered a separate category. These proteins are water-soluble but can aggregate and associate irreversibly with the lipid bilayer and become reversibly or irreversibly membrane-associated.
The most common tertiary structures are helix bundle and beta barrel. The portion of the membrane proteins that are attached to the lipid bilayer (see annular lipid shell) are consisting of hydrophobic amino acids only. This is done so that the peptide bonds' carbonyl and amine will react with each other instead of the hydrophobic surrounding. The portion of the protein that is not touching the lipid bilayer and is protruding out of the cell membrane are usually hydrophilic amino acids.
Membrane proteins 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.
Many of the successful membrane protein structures are characterized by X-ray crystallography and are very large structures in which the interactions with the membrane mimetic environments can be anticipated to be small in comparison to those within the protein structures. The small domains are particularly sensitive to the influence of membrane mimetic environments, with potential to lead to non-native structures. However, there are many sample preparation conditions that can be chosen for crystallization and for solution NMR. All membrane protein structural biology should be subjected to careful scrutiny; through a combination of structural methodologies it should be possible to achieve an understanding of the native functional state for membrane protein structures. Coevolution information has been successfully exploited for prediction of multiple large (membrane) protein structures.
Membrane proteins in genomes
A large fraction of all proteins are thought to be membrane proteins. For instance, about 1000 of the ~4200 proteins of E. coli are thought to be membrane proteins. The membrane localization has been confirmed for more than 600 of them experimentally. The localization of proteins in membranes can be predicted very reliably using hydrophobicity analyses of protein sequences, i.e. the localization of hydrophobic amino acid sequences.
- Integral membrane proteins
- Transmembrane proteins
- Peripheral membrane proteins
- Annular lipid shell
- Ion pump (biology)
- Carrier protein
- Ion channel
- Receptor (biochemistry) * List of MeSH codes (D12.776)
- Inner nuclear membrane proteins
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Membrane protein databases
- TCDB - Transporter Classification database, a comprehensive classification of transmembrane transporter proteins
- Orientations of Proteins in Membranes (OPM) database 3D structures of integral and peripheral membrane proteins arranged in the lipid bilayer
- Protein Data Bank of Transmembrane Proteins 3D models of all transmembrane proteins currently in PDB. Approximate positions of membrane boundary planes were calculated for each PDB entry.
- TransportDB Genomics-oriented database of transporters from TIGR
- Membrane PDB Database of 3D structures of integral membrane proteins and hydrophobic peptides with an emphasis on crystallization conditions
- List of transmembrane proteins of known 3D structure, incomplete list of transmembrane proteins currently used in to the Protein Data Bank
- Membrane targeting domains (MeTaDoR), a database of membrane targeting domains
- The Human Membrane Proteome - A comprehensive article covering the transmembrane protein component of the human proteome
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