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COPI‐coated vesicles that bud off the Golgi complex contain two major transmembrane proteins, p23 and p24. We have localized the protein at the Golgi complex and at COPI‐coated vesicles. Transport from the intermediate compartment (IC) to the Golgi can be blocked at 15°C, and under these conditions p24 accumulates in peripheral punctated structures identified as IC. Release from the temperature block leads to a redistribution of p24 to the Golgi, showing that p24, similar to p23, cycles between the IC and Golgi complex. Immunoprecipitations of p24 from cell lysates and from detergent‐solubilized Golgi membranes and COPI‐coated vesicles show that p24 and p23 interact with each other to form a complex. Transient transfection of p23 in HeLa cells shows that p23 and p24 colocalize in structures induced by the overexpression of p23. Taken together p24 interacts with p23 and constitutively cycles between the organelles of the early secretory pathway.

Several lines of evidence favour the hypothesis that intracellular biosynthetic protein transport in eukaryotes is mediated by non‐clathrin‐coated vesicles (for a review see Rothman and Orci, 1992). The vesicles have been isolated and a set of their surface proteins has been characterized as coat proteins (COPs). These COPs exist in the cytosol as a preformed complex, the coatomer, which was prior to this study known to contain six subunits: four (alpha‐, beta‐, gamma‐ and delta‐COP) with molecular weights between 160 and 58 kDa, and two additional proteins of approximately 36 and 20 kDa, epsilon‐ and xi‐COP. Here we describe a novel subunit of the coatomer complex, beta′‐COP. This subunit occurs in amounts stoichiometric to the established COPs both in the coatomer and in nonclathrin‐coated vesicles and shows homology to the beta‐subunits of trimeric G proteins.

In order to study the membrane topology and the possible function of the rat liver 22 kDa integral peroxisomal membrane protein (PMP 22) at a molecular level, we have cloned PMP 22 from a λgt11 expression library and sequenced its cDNA. Hydropathy analysis of the deduced primary structure indicates 4 putative transmembrane segments. The accessibility to exogenous aminopeptidase of PMP 22 in intact peroxisomes suggests that the N‐terminus faces the cytosol. A model of the topology of PMP 22 in the peroxisomal membrane is discussed. Homology studies revealed a striking similarity with the Mpv 17 gene product. Lack of this membrane protein causes nephrotic syndrome in mice.

In order to study the membrane topology and the possible function of the rat liver 22 kDa integral peroxisomal membrane protein (PMP 22) at a molecular level, we have cloned PMP 22 from a λgt11 expression library and sequenced its cDNA. Hydropathy analysis of the deduced primary structure indicates 4 putative transmembrane segments. The accessibility to exogenous aminopeptidase of PMP 22 in intact peroxisomes suggests that the N-terminus faces the cytosol. A model of the topology of PMP 22 in the peroxisomal membrane is discussed. Homology studies revealed a striking similarity with the Mpv 17 gene product. Lack of this membrane protein causes nephrotic syndrome in mice.

Several lines of evidence favour the hypothesis that intracellular biosynthetic protein transport in eukaryotes is mediated by non‐clathrin‐coated vesicles (for a review see Rothman and Orci, 1992). The vesicles have been isolated and a set of their surface proteins has been characterized as coat proteins (COPs). These COPs exist in the cytosol as a preformed complex, the coatomer, which was prior to this study known to contain six subunits: four (alpha‐, beta‐, gamma‐ and delta‐COP) with molecular weights between 160 and 58 kDa, and two additional proteins of approximately 36 and 20 kDa, epsilon‐ and xi‐COP. Here we describe a novel subunit of the coatomer complex, beta′‐COP. This subunit occurs in amounts stoichiometric to the established COPs both in the coatomer and in nonclathrin‐coated vesicles and shows homology to the beta‐subunits of trimeric G proteins.