Cytohesin-1, a cytosolic guanine nucleotide-exchange protein for ADP-ribosylation factor.
Cytohesin-1, a protein abundant in cells of the immune system, has been proposed to be a human homolog of the Saccharomyces cerevisiae Sec7 gene product, which is crucial in protein transport. More recently, the same protein has been reported to be a regulatory factor for the alphaLbeta2 integrin in lymphocytes. Overexpression of human or yeast ADP-ribosylation factor (ARF) genes rescues yeast with Sec7 defects, restoring secretory pathway function. ARFs, 20-kDa guanine nucleotide-binding proteins initially identified by their ability to stimulate cholera toxin ADP-ribosyltransferase activity and now recognized as critical components in intracellular vesicular transport, exist in an inactive cytosolic form with GDP bound (ARF-GDP). Interaction with a guanine nucleotide-exchange protein (GEP) accelerates exchange of GDP for GTP, producing the active ARF-GTP. Both soluble and particulate GEPs have been described. To define better the interaction between ARF and Sec7-related proteins, effects of cytohesin-1, synthesized in Escherichia coli, on ARF activity were evaluated. Cytohesin-1 enhanced binding of 35S-labeled guanosine 5'-[gamma-thio]triphosphate [35S]GTP[gammaS] or [3H]GDP to ARF purified from bovine brain (i.e., it appeared to function as an ARF-GEP). Addition of cytohesin-1 to ARF3 with [35S]GTP[gammaS] bound, accelerated [35S]GTP[gammaS] release to a similar degree in the presence of unlabeled GDP or GTP[gammaS] and to a lesser degree with GDP[betaS]; release was negligible without added nucleotide. Cytohesin-1 also increased ARF1 binding to a Golgi fraction, but its effect was not inhibited by brefeldin A (BFA), a drug that reversibly inhibits Golgi function. In this regard, it differs from a recently reported BFA-sensitive ARF-GEP that contains a Sec7 domain.
Meacci
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Pulmonary-Critical Care Medicine Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892-1590, USA.
Tsai
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SC
Adamik
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Moss
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Vaughan
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D020823:ADP-Ribosylation Factor 1; D020727:ADP-Ribosylation Factors; D000595:Amino Acid Sequence; D020126:Brefeldin A; D015815:Cell Adhesion Molecules / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D002461:Cell Line, Transformed; D003001:Cloning, Molecular; D003517:Cyclopentanes / Q000494:pharmacology; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D005800:Genes, Fungal / Q000235:genetics; D020662:Guanine Nucleotide Exchange Factors; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D011485:Protein Binding; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D016415:Sequence Alignment; D017421:Sequence Analysis
D015815:Cell Adhesion Molecules; D003517:Cyclopentanes; D005656:Fungal Proteins; D020662:Guanine Nucleotide Exchange Factors; C066755:Sec7 guanine nucleotide exchange factors; C100723:cytohesin-1; D006153:Guanosine Diphosphate; D020126:Brefeldin A; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins; D020823:ADP-Ribosylation Factor 1; D020727:ADP-Ribosylation Factors