Interactions between Ras and Raf: key regulatory proteins in cellular transformation

Ras proteins function during cell growth and development as essential, plasma membrane-bound signaling proteins. Current evidence suggests that Ras is part of a signal transduction chain extending from extracellular signals to transcriptional regulation in the nucleus. Growth factor and cytokine activation of many tyrosine kinase and kinase-linked receptors recruits many proteins to the plasma membrane including Ras-specific guanine nucleotide releasing proteins (GNRP). Under the influence of a GNRP, Ras proteins bind GTP, resulting in activation of the Ras signal. The GTP-bound form of Ras is capable of interacting directly with RasGAP, neurofibromin, and the Raf kinases. Although believed to be endowed with some signaling capacity, RasGAP and neurofibromin act primarily to negatively regulate Ras. Based upon genetic and biochemical studies in a variety of diverse organisms, the Raf kinases are considered the primary targets of Ras signaling. Activation of the Raf kinases is the first step in a cascade of multiple protein kinases, including Mek, Erk1, and Erk2. We are attempting to understand structurally how activated Ras proteins interact specifically with Raf kinases to induce the downstream signals necessary for cell division. Using mutagenesis, peptide epitope scanning, and in vitro reconstitution of protein interactions, we have identified specific sites of association between the Ras-GTP and c-Raf-1 proteins. The interaction between these contact points is essential for the plasma membrane localization of Raf, which ultimately leads to kinase activation. The formation of this protein complex is negatively regulated by protein kinase A (PKA) through phosphorylation of the c-Raf-1 N-terminus. Phosphorylation of c-Raf-1 serine 43 is believed to cause an N-terminal cap structure to cover the Ras docking site.