ARD-1, A 64-KDA GUANINE NUCLEOTIDE-BINDING PROTEIN WITH A CARBOXYL-TERMINAL ADP-RIBOSYLATION FACTOR DOMAIN

Clones referred to as ARD 1 were isolated from human and rat cDNA libr aries. ARD 1 genes encode a putative 64-kDa protein that contains an 1 8-kDa ADP-ribosylation factor (ARF) domain at the carboxyl terminus an d is much larger than the other monomeric approximately 20-kDa guanine nucleotide-binding ARF proteins thus far identified. ARD 1 mRNAs of 3 .7 and 4.1 kilobases were detected in all rat tissues as well as in mo use and rabbit brain, human fibroblasts, and human neuroblastoma cells but not in HL-60 cells. Based on sequence identities, ARD 1 is highly conserved between rat and human. The ARF domain of ARD 1 contains the consensus sequences believed to be involved in guanine nucleotide bin ding, which are conserved in the ARFs and other GTP-binding proteins. Recombinant ARD 1 or the ARF domain of ARD 1, which lacks the 15 amino acids corresponding to the amino-terminal regions of ARFs stimulated, in a GTP-dependent manner, cholera toxin ADP-ribosyltransferase activ ity in the presence of 0.3% Tween 20. It had no effect in the presence of SDS, dimyristoylphosphatidylcholine/cholate, or cardiolipin. These observations are consistent with the conclusion that the amino-termin al region of ARF proteins is not required for activation of cholera to xin. In addition, the characteristic features of ARF proteins may be f ound as domains of larger mammalian proteins.