Structural elements of ADP-ribosylation factor 1 required for functional interaction with cytohesin-1

ADP-ribosylation factor 1 (ARF1) is a 20-kDa guanine nucleotide-binding pro tein involved in vesicular trafficking. Conversion of inactive ARF-G;DP to active ARF-GTP is catalyzed by guanine nucleotide exchange proteins such as cytohesin-1. Cytohesin-1 and its Sec7 domain (C-1Sec7) exhibit guanine nuc leotide exchange protein activity with ARF1 but not ARF-like protein 1 (ARL 1), which is 57% identical in amino acid sequence. With chimeric proteins c omposed of ARF1 (F) and ARL1 (L) sequences we identified three structural e lements responsible for this specificity. Cytohesin-1 increased [S-35]guano sine 5'-(gamma-thio)triphosphate binding to L28/F first 28 residues of L, r emainder Fl and to a much lesser extent F139L, and mut13F139/L (F139/L with random sequence in the first 13 positions) but not Delta 13ARF1 that lacks the first 13 amino acids; therefore, a nonspecific ARF N terminus was requ ired for cytohesin-1 action. The N terminus was not, however, required for that of C-1Sec7. Both C-1Sec7 and cytohesin-1 effectively released guanosin e 5'-(gamma-thio)triphosphate from ARF1, but only C-1Sec7 displaced the non hydrolyzable GTP analog bound to mut13F139/L, again indicating that structu re in addition to the Sec7 domain is involved in cytohesin-1 interaction. S ome element(s) of the C-terminal region is also involved, because replaceme nt of the last 42 amino acids with ARL sequence in F139L decreased markedly the interaction with cytohesin-1. Participation of both termini is consist ent with the crystallographic structure of ARF in which the two terminal a- helices are in close proximity. ARF1 residues 28-50 are also important in t he interaction with cytohesin-1; replacement of Lys-38 with Gin, the corres ponding residue in ARL1, abolished the ability to serve as substrate for cy tohesin-1 or C-1Sec7. These studies have defined multiple structural elemen ts in ARF1, including switch 1 and the N and C termini, that participate in functional interactions with cytohesin-1 (or its catalytic domain C-1Sec7) , which were not apparent from crystallographic analysis.