Characterization of a guanine nucleotide dissociation stimulator for a ras‐related GTPase

ras‐related GTPases participate in signaling for a variety of cellular processes. The GTPases cycle between a GTP‐bound active state and a GDP‐bound inactive state. This cycling is partially controlled by guanine nucleotide dissociation stimulators (GDS, also known as exchange factors). We report on the molecular cloning of cDNAs encoding a new mammalian GDS protein, using sequences derived from the yeast ras GDS proteins as probes. The encoded protein stimulates the dissociation of guanine nucleotides from the ras‐related ralA and ralB GTPases at a rate at least 30‐fold faster than the intrinsic nucleotide dissociation rate. This new GDS, ralGDS, is at least 20‐fold more active on the ralA and ralB GTPases than on any other GTPase tested, including other members of the ras family (H‐ras, N‐ras, K‐ras, R‐ras, rap1a and rap2), members of the rho family (rhoA, rhoB and CDC42‐Hs) and members of the rab family (rab3a and ypt1). While the ralGDS protein is phosphorylated on serine residues, we find no evidence that phosphorylation affects the activity of insect cell‐expressed ralGDS towards the ralA or ralB GTPase. The 3600 nucleotide ralGDS mRNA and the 115 kDa protein were found in all tissues and cell lines examined.