CHARACTERIZATION OF A 76 KDA ENDOSOMAL, MULTISPANNING MEMBRANE-PROTEIN THAT IS HIGHLY CONSERVED THROUGHOUT EVOLUTION

We report here the identification and characterization of a human 76 k Da membrane protein that is found predominantly in endosomes. This pro tein is related to the Saccharomyces cerevisiae EMP70 gene product, a precursor protein whose 24 kDa cleavage product (p24a) was found in ye ast endosome-enriched membrane fractions (Singer-Kruger et al., 1993. J. Biol. Chem. 268, 14376-14386). Northern blot analysis indicated tha t p76 mRNA is highly expressed in human pancreas but could be detected in all tissues examined. p76 is highly conserved throughout evolution , as related proteins have also been detected in Caenorhabditis elegan s and Arabidopsis thaliana. This family of proteins has a relatively d ivergent, hydrophilic N-terminal domain and a well-conserved, highly h ydrophobic C-terminal domain which contains nine potential membrane-sp anning domains. Transiently expressed, myc-tagged human p76 appears to be localized to endosomes by virtue of its apparent colocalization wi th transferrin receptors and some mannose 6-phosphate receptors. Furth ermore, p76 adopts a type-I topology within the membrane, with its hyd rophilic N-terminus facing the lumen of cytoplasmic membranes. The str uctural features of p76 suggest that it may function as a channel or s mall molecule transporter in intracellular compartments throughout phy logeny. (C) 1998 Elsevier Science B.V. All rights reserved.