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 kDa membrane protein that is found predominantly in endosomes. This protein is related to the Saccharomyces cerevisiae EMP70 gene product, a precursor protein whose 24 kDa cleavage product (p24a) was found in yeast endosome-enriched membrane fractions. Northern blot analysis indicated that 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 elegans and Arabidopsis thaliana. This family of proteins has a relatively divergent, hydrophilic N-terminal domain and a well-conserved, highly hydrophobic C-terminal domain which contains nine potential membrane-spanning domains. Transiently expressed, myc-tagged human p76 appears to be localized to endosomes by virtue of its apparent colocalization with transferrin receptors and some mannose 6-phosphate receptors. Furthermore, p76 adopts a type-I topology within the membrane, with its hydrophilic N-terminus facing the lumen of cytoplasmic membranes. The structural features of p76 suggest that it may function as a channel or small molecule transporter in intracellular compartments throughout phylogeny.