THE MIP FAMILY OF INTEGRAL MEMBRANE CHANNEL PROTEINS - SEQUENCE COMPARISONS, EVOLUTIONARY RELATIONSHIPS, RECONSTRUCTED PATHWAY OF EVOLUTION, AND PROPOSED FUNCTIONAL-DIFFERENTIATION OF THE 2 REPEATED HALVES OF THE PROTEINS

The major intrinsic protein (MIP) of the bovine lens fiber cell membrane was the first member of the MIP family of proteins to be sequenced and characterized. It is probably a homotetramer with transmembrane channel activity that plays a role in lens biogenesis or maintenance. The polypeptide chain of each subunit may span the membrane six times, and both the N- and C-termini face the cell cytoplasm. Eighteen sequenced or partially sequenced proteins from bacteria, yeast, plants, and animals have now been shown to be members of the MIP family. These proteins appear to function in (1) metazoan development and neurogenesis (MIP and BIB), (2) water transport across the human erythrocyte membrane (ChEP), (3) communication between host plant cells and symbiotic nitrogen-fixing bacteria (NOD), (4) transport across the tonoplast membrane during plant seed development (alpha-TIP), (5) water stress-induced resistance to desiccation in plants (Wsi-TIP), (6) suppression of a genetic growth defect on fermentable sugars in yeast (FPS 1), and (7) transport of glycerol across bacterial cell membranes (GlpF). One other sequenced member of the MIP family (ORF1 of Lactococcus lactis) has no known physiological function. The biochemical functions of the eukaryotic proteins are not well established. Computer analyses have revealed that the first and second halves of all MIP family proteins probably arose by a tandem, intragenic, duplication event. Thus, the primary structure of putative transmembrane helices 1 to 3 is similar to that of putative transmembrane helices 4 to 6 even though they are of opposite orientation in the membrane. Among the most conserved residues in these two repeated halves are a membrane-embedded glutamate (E) in helices 1 and 4, an asparagine-proline-alanine (NPA) sequence in the loops between helices 2 and 3 (cytoplasmically localized) and helices 5 and 6 (extracellularly localized), and a glycine within helices 3 and 6. Statistical analyses suggest that the two halves of these proteins have evolved to serve distinct functions: the first half is more important for the generalized or common functions of these proteins, while the second half of these proteins is more differentiated to provide specific or dissimilar functions of the proteins. The apparent origin of MIP family proteins by duplication of a three-spanner precursor protein suggests an evolutionary origin distinct from other transport proteins with six transmembrane spanners. Based on the phylogenetic tree for the 18 sequenced members of the MIP family, we propose that a single, primordial gene arose in prokaryotes shortly before the emergence of eukaryotes, that this gene was vertically transmitted to the principal eukaryotic kingdoms, and that subsequent gene duplication and divergence events gave rise to kingdom-related subfamilies or clusters of the MIP family.