Direct interaction of Ras and the amino-terminal region of Raf-1 in vitro

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Abstract
THE Ras proteins are key regulators of the growth of eukaryotic cells, but their direct target enzymes, or 'effectors', are unknown1. The protein encoded by the c-raf-1 proto-oncogene is thought to function downstream of p21ras because disruption of Raf blocks signalling by Ras in a number of systems2–5. Here we report that the amino-terminal cysteine-rich regulatory region of p74c-raf-1 expressed as a glutathione-S-transferase (GST) fusion protein binds directly to Ras with relatively high affinity (50 nM). The binding is strictly dependent on the Ras protein being in the active GTP-bound conformation rather than the inactive GDP-bound state. Raf–GST interacts with wild-type and oncogenic Ras (Val 12) but fails to interact with a biologically inert effector mutant of Ras (Ala 38) and a dominant negative mutant (Asn 17). A peptide based on the effector region of Ras inhibits the interaction. Raf–GST acts as a potent competitive inhibitor of the GTPase-activating proteins p120GAPand neurofibromin. In addition, Raf itself displays weak GTPase-stimulating activity towards Ras. It is therefore likely that Raf is a direct effector of Ras.