Specific functional interaction of human cytohesin-1 and ADP-ribosylation factor domain protein (ARD1).

Activation of ADP-ribosylation factors (ARFs) is mediated by guanine nucleotide-exchange proteins, which accelerate conversion of inactive ARF-GDP to active ARF-GTP. ARF domain protein (ARD1), a 64-kDa GTPase with a C-terminal ADP-ribosylation factor domain, is localized to lysosomes and the Golgi apparatus. When ARD1 was used as bait to screen a human liver cDNA library using the yeast two-hybrid system, a cDNA for cytohesin-1, a approximately 50-kDa protein with ARF guanine nucleotide-exchange protein activity, was isolated. In this system, ARD1-GDP interacted well with cytohesin-1 but very poorly with cytohesin-2. In agreement, cytohesin-1, but not cytohesin-2, markedly accelerated [(35)S]guanosine 5'-3-O-(thio)triphosphate binding to ARD1. The effector region of the ARF domain of ARD1 appeared to be critical for the specific interaction with cytohesin-1. Replacement of single amino acids in the Sec7 domains of cytohesin-1 and -2 showed that residue 30 is critical for specificity. In transfected COS-7 cells, overexpressed ARD1 and cytohesin-1 were partially colocalized, as determined by confocal fluorescence microscopy. It was concluded that cytohesin-1 is likely to be involved in ARD1 activation, consistent with a role for ARD1 in the regulation of vesicular trafficking.