ARP is a plasma membrane-associated Ras-related GTPase with remote similarity to the family of ADP-ribosylation factors

The human and rat homologues of a novel Ras-related GTPase with unique structural features were cloned by polymerase chain reaction amplification and cDNA library screening. Their deduced amino acid sequences are highly homologous (97% identical amino acids; 88.3% identical nucleotides within the coding region) and comprise all six of the conserved motifs presumably involved in GTP binding. Because the sequences exhibit some similarity with members of the ADP-ribosylation factor (ARF) family (33% identity with ADP-ribosylation factor 1 (ARF1), 39% identity with ARF-like 3), the protein was designated ARP (ARF-related protein). In contrast to all other members of the ARF family, ARP lacks the myristoylation site at position 2 and comprises an insertion of 8 amino acids in the region between PM1 and PM2. mRNA was found in most rat tissues examined (skeletal muscle, fat, liver, kidney, spleen, testis, adrenals, ovary, thymus, intestine, and lung). Western blot analysis with antiserum against recombinant ARP showed a 25-kDa protein in membranes from rat liver, testis, and kidney. Thus, the protein appears to be posttranslationally modified for membrane anchoring. Unlike ARF, the ARP immunoreactivity was detected in plasma membranes but not in cytosol of fractionated 3T3-L1 cells. Recombinant ARP exhibited specific and saturable GTP gamma S (guanosine 5'-3-O-(thio)triphosphate) binding and, unlike ARF isotypes, GTPase activity in the absence of tissue extracts or phospholipids. Thus, the structural and functional characteristics of ARP indicate that it represents a novel subtype of GTPases, presumably exerting a unique function and possibly involved in plasma membrane-related signaling events.