A panel of mAbs was elicited against intracellular membrane fractions from rat pancreas. One of the antibodies reacted with a 95-kDa protein that localizes primarily to the Golgi complex or the endoplasmic reticulum (ER), depending on cell type. The corresponding cDNA was cloned and sequenced and found to encode a protein of 97.6 kDa that we call GERp95 (Golgi ER protein 95 kDa). The protein copurifies with intracellular membranes but does not contain hydrophobic regions that could function as signal peptides or transmembrane domains. Biochemical analysis suggests that GERp95 is a cytoplasmically exposed peripheral membrane protein that exists in a protease-resistant complex. GERp95 belongs to a family of highly conserved proteins in metazoans and Schizosaccharomyces pombe. It has recently been determined that plant and Drosophila homologues of GERp95 are important for controlling the differentiation of stem cells (Bohmert et al., 1998;Cox et al., 1998;Moussian et al., 1998). In Caenorhabditis elegans, there are at least 20 members of this protein family. To this end, we have used RNA interference to show that the GERp95 orthologue in C. elegans is important for maturation of germ-line stem cells in the gonad. GERp95 and related proteins are an emerging new family of proteins that have important roles in metazoan development. The present study suggests that these proteins may exert their effects on cell differentiation from the level of intracellular membranes. INTRODUCTIONThe cytoplasm of eukaryotic cells is partitioned into more than a dozen membrane-bound organelles. Compartmentalization serves to increase the efficiencies of cellular processes by controlling the spatial and temporal interactions of proteins, nucleic acids and lipids. The endoplasmic reticulum (ER) and Golgi complex play central roles in the biogenesis and operational fidelity of eukaryotic cells by orchestrating the synthesis and movement of proteins and lipids (Hurtley and Helenius, 1989;Narula et al., 1992;Sitia and Meldolesi, 1992;Bergeron et al., 1994;Hammond and Helenius, 1995;Farquhar and Hauri, 1997;Hauri and Schweizer, 1997;Farquhar and Palade, 1998). It is now clear that these two organelles are directly involved in processes that affect cellular differentiation because mistargeting and/or altered expression of resident proteins of the ER and Golgi complex can have profound effects on cell growth, morphology, and tumorigenicity. Moreover, cellular defects at the ER/Golgi level underlie the pathophysiology of many human diseases such as familial hypercholesterolemia, polycystic kidney disease, Tangier disease, cystic fibrosis, mucopolysaccharidosis types I, VI, and VII, progeroid syndrome, and many others (for review, see Brooks, 1997;Gonatas, 1997).We are interested in identifying novel ER-and Golgiassociated proteins that are involved in the function of these organelles. The approach we adopted was to generate a panel of mAbs against ER/Golgi membrane fractions so that they may be used to identify and then immunoaffinity purify the...