A MAJOR TRANSMEMBRANE PROTEIN OF GOLGI-DERIVED COPI-COATED VESICLES INVOLVED IN COATOMER BINDING

Formation of non-clathrin-coated vesicles requires the recruitment of several cytosolic factors to the Golgi membrane. To identify membrane proteins involved in this budding process, a highly abundant type I tr ansmembrane protein (p23) was isolated from mammalian Golgi-derived CO PI-coated vesicles, and its cDNA was cloned and sequenced. It belongs to the p24 family of proteins involved in the budding of transport ves icles (Stamnes, M.A., M.W. Craighead, M.H. Hoe, N. Lampen, S. Geromano s, P. Tempst, and J.E. Rothman. 1995. Proc. Natl. Acad. Sci. USA. 92:8 011-8015). p23 consists of a large NH2-terminal luminal domain and a s hort COOH-terminal cytoplasmic tail (-LRRFFKAKKLIE-CO2-) that shows si milarity, but not identity, with the sequence motif -KKXX-CO2-, known as a signal for retrieval of escaped ER-resident membrane proteins (Ja ckson, M.R., T. Nilsson, and P.A. Peterson. 1990. EMBO (fur. Mel. Biol . Organ.) J. 9:3153-3162; Nilsson, T., M. Jackson, and P.A. Peterson. 1989. Cell. 58:707-718). The cytoplasmic tail of p23 binds to coatomer with similar efficiency as known KKXX motifs. However, the p23 tail d iffers from the KKXX motif in having an additional motif needed for bi nding of coatomer. p23 is localized to Golgi cisternae and, during ves icle formation, it concentrates into COPI-coated buds and vesicles. Bi ochemical analysis revealed that p23 is enriched in vesicles by a fact or of similar to 20, as compared with the donor Golgi fraction, and is present in amounts stoichiometric to the small GTP-binding protein AD P-ribosylation factor (ARF) and coatomer. From these data we conclude that p23 represents a Golgi-specific receptor for coatomer involved in the formation of COPI-coated vesicles.