Guanine nucleotide exchange on ADP-ribosylation factors catalyzed by cytohesin-1 and its Sec7 domain.

ADP-ribosylation factors (ARFs) are 20-kDa guanine nucleotide-binding proteins that require specific guanine nucleotide-exchange proteins (GEPs) to accelerate the conversion of inactive ARF-GDP to active ARF-GTP. Cytohesin-1, a 46-kDa ARF GEP, contains a central Sec7 domain of 188 amino acids similar in sequence to a region of the yeast Sec7 protein. Cytohesin-1 and its 22-kDa Sec7 domain (C-1 Sec7), synthesized in Escherichia coli, were assayed with recombinant non-myristoylated ARFs and related proteins to compare their GEP activities. Both were effective with native mammalian ARFs 1 and 3. Cytohesin-1 accelerated GTPgammaS (guanosine 5'-3-O-(thio)triphosphate) binding to recombinant human ARF1 (rARF1), yeast ARF3, and ARD1 (a 64-kDa guanine nucleotide-binding protein containing a C-terminal ARF domain). In contrast, C-1 Sec7 enhanced GTPgammaS binding to recombinant human ARFs 1, 5, and 6; yeast ARFs 1, 2, and 3; ARD1; two ARD1 mutants that contain the ARF domain; and Delta13ARF1, which lacks the N-terminal alpha-helix. Neither C-1 Sec7 nor cytohesin-1 increased GTPgammaS binding to human ARF-like ARL proteins 1, 2, and 3. Thus, ARLs, initially differentiated from ARFs because of their inability to activate cholera toxin, differ also in their failure to interact functionally with C-1 Sec7 or cytohesin-1. As C-1 Sec7 was much less substrate-specific than cytohesin-1, it appears that structure outside of the Sec7 domain is important for ARF specificity. Data obtained with mutant ARF constructs are all consistent with the conclusion that the ARF N terminus is an important determinant of cytohesin-1 specificity.