The coatomer is a cytosolic protein complex that reversibly associates with Golgi membranes and is Implicated in modulating Golgi membrane transport.

Hydrolysis of guanosine triphosphate (GTP) by the small guanosine triphosphatase (GTPase) adenosine diphosphate ribosylation factor-1 (ARF1) depends on a GTPase-activating protein (GAP). A complementary DNA encoding the ARF1 GAP was cloned from rat liver and predicts a protein with a zinc finger motif near the amino terminus. The GAP function required an intact zinc finger and additional amino-terminal residues. The ARF1 GAP was localized to the Golgi complex and was redistributed into a cytosolic pattern when cells were treated with brefeldin A, a drug that prevents ARF1-dependent association of coat proteins with the Golgi. Thus, the GAP is likely to be recruited to the Golgi by an ARF1-dependent mechanism.

Treatment of pigeon erythrocyte membranes with cholera toxin and NAD+ enhan the GTP stimulation and suppressed the F activation of the adenylate cyclase [ATP pyrophosphatemlyase (cyclizing), EC 4.6.11. In the presence of NAD+ labeled with aP in the AMP moiety the toxin catalyzed the covalent incorporation of radioactivity into membrane proteins with molecular weights (Mrs) of 200,000, 86,000, and 42,000 Extraction of toxin-treated membranes with Lubrol PX followed by affinity chromatography on a GTP-Sepharose column resulted in a 200-fold purification of the 42,000-M, labeled protein and in its complete separation from the other labeled proteins. The fraction containing the purified GTP-binding component from toxin-treated membranes conferred an enhanced GP-stimulated activity on adenylate cyclase solubilized from nontreated membranes. Likewise, the addition of GTPbinding fraction from nontreated membranes to an enzyme solubilized from toxin-treated membranes restored F-stimulation of the adenylate cyclase. The toxin-induced modification of adenylate cyclase and the incorration of radioactivity into the 42,000-M, protein were partially reversed upon incubation with toxin and nicotinamide at pH (1. The results indicate that cholera toxin affects the adenylate cyclase system by catalyzing an ADP-ribosylation of the 42,000-Mr component bearing the guanyl nucleotide regulatory site.

a b s t r a c tTransport of membranes and proteins in eukaryotic cells is mediated by vesicular carriers. Here we review the biogenesis and functions of COPI vesicles, carriers that operate in the early secretory pathway. We focus on mechanisms mediating coat recruitment, uptake of cargo, vesicle budding and fission, and finally dissociation of the coat. In this context, recent findings on the interplay between machinery and auxiliary proteins in COPI vesicle formation and function will be discussed. Specifically, we will weigh the pros and cons of recent data on roles of the small GTP binding protein Arf1, of Arf1GAPs, and lipids during COPI carrier formation.