IDENTIFICATION AND CHARACTERIZATION OF RAL-BINDING PROTEIN-1, A POTENTIAL DOWNSTREAM TARGET OF RAL GTPASES

Ral proteins constitute a distinct family of Ras-related GTPases, Alth ough similar to Ras in amino acid sequence, Ral proteins are activated by a unique nucleotide exchange factor and inactivated by a distinct GTPase-activating protein, Unlike Ras, they fail to promote transforme d foci when activated versions are expressed in cells, To identify dow nstream targets that might mediate a Ral-specific function, we used a Saccharomyces cerevisiae-based interaction assay to clone a novel cDNA that encodes a Ral-binding protein (RalBP1), RalBP1 binds specificall y to the active GTP-bound form of RalA and not to a mutant Ral with a point mutation in its putative effector domain, In addition to a Ral-b inding domain, RalBP1 also contains a Rho-GTPase-activating protein do main that interacts preferentially with Rho family member CDC42, Since CDC42 has been implicated in bud site selection in S. cerevisiae and filopodium formation in mammalian cells, Ral may function to modulate the actin cytoskeleton through its interactions with RalBP1.