The MIP family of integral membrane channel proteins

Publisher Summary This chapter illustrates that all members of the major intrinsic proteins (MIP) family serve a common function in transmembrane transport of small molecules, although each class of these proteins may exhibit distinctive specificities and characteristics. Some of these proteins are regulated by protein kinase-mediated phosphorylation, whereas others are posttranslationally modified by palmitoylation. Some of these proteins have distinctive N- and/or C-terminal extensions that are not homologous to other proteins in the database. Big brain (BIB) and fermentable sugar defect suppressor (FPS) have extensive hydrophilic N- and C-terminal regions of this type. The degrees of conservation of the proposed sites of phosphorylation between members of the family are also discussed. The two halves of all MIP family proteins are derived from a common ancestral gene, half as big as the present genes, and this ancestral gene tandemly duplicated internally to give rise to the primordial gene of the MIP family proteins. It is presumed that this event occurred in prokaryotes, well before divergence of the species. MIP family proteins are roughly related to each other, as are the organisms in which they are found. As only one such gene has been found in any one bacterium, but several have been found in single eukaryotes, studies propose that a single MIP family gene was transmitted vertically from the prokaryotic ancestor to each of the eukaryotic kingdoms, and that these genes then duplicated and diverged within eukaryotes to yield present MIP protein subfamilies.