IDENTIFICATION OF RAP1 AS A TARGET FOR THE CRK SH3 DOMAIN-BINDING GUANINE NUCLEOTIDE-RELEASING FACTOR C3G

C3G, which was identified as a Crk SH3 domain-binding guanine nucleoti de-releasing factor, shows sequence similarity to CDC25 and Sos family proteins (S. Tanaka, T. Morishita, Y. Hashimoto, S. Hattori, S. Nakam ura, M. Shibuya, K. Matuoka, T. Takenawa, T. Kurata, K. Nagashima, and M. Matsuda, Proc. Natl. Acad. Sci. USA 91:3443-3447, 1994). The subst rate specificity of C3G was examined by in vitro and in vivo experimen ts. C3G markedly stimulated dissociation of bound GDP from Rap1B but m arginally affected the same reaction of other Pas family proteins (Ha- Ras, N-Ras, and RalA). C3G also stimulated binding of GTP-gamma S [gua nosine 5'-3-O-(thio)triphosphate] to Rap1B. When C3G and Rap1A were ex pressed in COS7 cells, marked accumulation of the active GTP-bound for m of Rap1A was observed, while Sos was not effective in the activation of RaplA. These results clearly show that C3G is an activator for Rap 1. Furthermore, expression of C3G with a membrane localization signal in a v-Ki-ras transformant, DT, induced a reversion of the cells to th e hat form, possibly through the activation of endogenous Rap1.