Major intrinsic proteins
| Major intrinsic protein | |||||||||
|---|---|---|---|---|---|---|---|---|---|
Structure of a glycerol-conducting channel.
|
|||||||||
| Identifiers | |||||||||
| Symbol | MIP | ||||||||
| Pfam | PF00230 | ||||||||
| InterPro | IPR000425 | ||||||||
| PROSITE | PDOC00193 | ||||||||
| SCOP | 1fx8 | ||||||||
| SUPERFAMILY | 1fx8 | ||||||||
| TCDB | 1.A.8 | ||||||||
| OPM superfamily | 7 | ||||||||
| OPM protein | 1z98 | ||||||||
| CDD | cd00333 | ||||||||
|
|||||||||
| Available protein structures: | |
|---|---|
| Pfam | structures |
| PDB | RCSB PDB; PDBe; PDBj |
| PDBsum | structure summary |
Major intrinsic proteins comprise a large superfamily of transmembrane protein channels that are grouped together on the basis of homology. The MIP superfamily includes three subfamilies: aquaporins, aquaglyceroporins and S-aquaporins.
The phylogeny of insect MIP family channels has been published.
There are two families that belong to the MIP Superfamily.
The MIP family is large and diverse, possessing thousands of members that form transmembrane channels. These channel proteins function in transporting water, small carbohydrates (e.g., glycerol), urea, NH3, CO2, H2O2 and ions by energy-independent mechanisms. For example, the glycerol channel, FPS1p of Saccharomyces cerevisiae mediates uptake of arsenite and antimonite. Ion permeability appears to occur through a pathway different than that used for water/glycerol transport and may involve a channel at the 4 subunit interface rather than the channels through the subunits. MIP family members are found ubiquitously in bacteria, archaea and eukaryotes. Phylogenetic clustering of the proteins is primarily based according to phylum of the organisms of origin, but one or more clusters are observed for each phylogenetic kingdom (plants, animals, yeast, bacteria and archaea). MIPs are classified into five subfamilies in higher plants, including plasma membrane (PIPs), tonoplast (TIPs), NOD26-like (NIPs), small basic (SIPs) and unclassified X (XIPs) intrinsic proteins. One of the plant clusters includes only tonoplast (TIP) proteins, while another includes plasma membrane (PIP) proteins.
The Major Intrinsic Protein (MIP) of the human lens of the eye (Aqp0), after which the MIP family was named, represents about 60% of the protein in the lens cell. In the native form, it is an aquaporin (AQP), but during lens development, it becomes proteolytically truncated. The channel, which normally houses 6-9 water molecules, becomes constricted so only three remain, and these are trapped in a closed conformation. These truncated tetramers form intercellular adhesive junctions (head to head), yielding a crystalline array that mediates lens formation with cells tightly packed as required to form a clear lens. Lipids crystallize with the protein. Ion channel activity has been shown for Aquaporins 0, 1, and 6, Drosophila 'Big Brain' (bib) and plant Nodulin-26. Roles of aquaporins in human cancer have been reviewed as have their folding pathways. AQPs may act as transmembrane osmosensors in red cells, secretory granules and microorganisms. MIP superfamly proteins and variations of their selectivity filters have been reviewed.
...
Wikipedia