Similarities in function and gene structure of cytohesin-4 and cytohesin-1, guanine nucleotide-exchange proteins for ADP-ribosylation factors

Activation of ADP-ribosylation factors (ARFs), similar to 20-kDa GTPases th at are inactive in the GDP-bound form, depends on guanine nucleotide-exchan ge proteins (GEPs) to accelerate GTP binding, A novel ARF GEP, designated c ytohesin-4, was cloned from a human brain cDNA library. Deduced amino acid sequence of the 47-kDa protein contains the same structural components pres ent in cytohesin -1, -2, and -3, including an similar to 200-amino acid Sec 7 domain with an similar to 100-residue pleckstrin homology domain near the C terminus. The Sec7 domain sequence is 77% identical to those of other cy tohesins. Structures of the cytohesin-4 and cytohesin-1 genes were remarkab ly similar, except for an extra 3-base pair (GAG) exon present in cytohesin -1. Two mRNAs with and without the 3-base pair sequence were found in brain in different ratios for cytohesin-1, -2, and -3 but not cytohesin-4, Recom binant cytohesin-4 stimulated guanosine 5'-3-O-(thio)triphosphate binding b y human ARF1 and ARF5 but not ARF6. Like other cytohesins and unlike the si milar to 200-kDa ARP GEPs, it was not inhibited by brefeldin A A cytohesin- 4 mRNA of similar to 3.7 kilobases, abundant in leukocytes, was not detecte d in most tissues. Among separated populations of blood cells, similar to 9 0% of CD33(+) (monocytes), 80% of CD2(+) (NK/T), and 10-20% of CD19(+) (B) cells contained cytohesin-4 mRNA by in situ hybridization, Thus, in gene st ructure and brefeldin A-insensitive GEP activity, cytohesin-4 resembles oth er cytohesins, but its tissue distribution differs considerably, consistent with a different specific function.