InterPro domain: IPR000425

The major intrinsic protein (MIP) family is large and diverse, possessing over 100 members that form transmembrane channels. These channel proteins function in water, small carbohydrate (e.g., glycerol), urea, NH3, CO2 and possibly ion transport, by an energy independent mechanism. They are found ubiquitously in bacteria, archaea and eukaryotes.

The MIP family contains two major groups of channels: aquaporins and glycerol facilitators. The known aquaporins cluster loosely together as do the known glycerol facilitators. MIP family proteins are believed to form aqueous pores that selectively allow passive transport of their solute(s) across the membrane with minimal apparent recognition. Aquaporins selectively transport water (but not glycerol) while glycerol facilitators selectively transport glycerol but not water. Some aquaporins can transport NH3 and CO2. Glycerol facilitators function as solute nonspecific channels, and may transport glycerol, dihydroxyacetone, propanediol, urea and other small neutral molecules in physiologically important processes. Some members of the family, including the yeast FPS protein and tobacco NtTIPA may transport both water and small solutes.

The structures of various members of the MIP family have been determined by means of X-ray diffraction [ 1 , 2 , 3 ], revealing the fold to comprise a right-handed bundle of 6 transmembrane (TM) alpha-helices [ 4 , 4 , 4 ]. Similarities in the N-and C-terminal halves of the molecule suggest that the proteins may have arisen through tandem, intragenic duplication of an ancestral protein that contained 3 TM domains [ 4 ].

1. Structural basis of water-specific transport through the AQP1 water channel. Nature 414, 872-8
2. Crystallization and preliminary X-ray crystallographic analysis of water channel AQP1. Acta Crystallogr. D Biol. Crystallogr. 56, 1198-200
3. Structure of a glycerol-conducting channel and the basis for its selectivity. Science 290, 481-6
4. Tandem sequence repeats in transmembrane channel proteins. Trends Biochem. Sci. 16, 170-1