Partial purification of a GTPase-activating protein for rap2b from bovine brain membranes

Rap2b is a ras-related GTP-binding protein isolated from a human platelet cDNA library. It shares 90% similarity to the previously described rap2a and is closely related to rap1a (Krev-1, smgp21), which has been shown to possess reversion of transformation activity in Kirsten ras transformed 3T3 cells. In this study we have partially purified a protein from bovine brain membranes which stimulates the GTPase activity of rap2b. This rap2b GTPase-activating protein (GAP) activity is not immunoreactive with antibodies specific for rap1 GAP or ras GAP, yet displays limited GTPase stimulatory activity toward rap1. This result differs from the previously described rap1 GAP which is highly specific for rap1. When the rap2 GAP activity is analyzed by coomassie staining, an enrichment of a approximately 55 kDa protein is observed providing further evidence of a distinct rap2 GAP.