Interaction of Raf Kinase with Ras Protein

Genetic and biochemical investigations point to a function as a central effector of signal transduction via Ras proteins for the Raf kinase. Raf kinase is immediately downstream from Ras protein in the signal chain. The activated GTP form of Ras protein interacts in a specific manner with Raf kinase and thus mediates membrane localization of Raf kinase. Consequently, the protein kinase activity of Raf kinase is stimulated and the signal is transmitted further via the protein kinase cascade of the MAP kinase pathway. 

Signal transduction between Ras protein and Raf kinase is based on a specific interaction of the two proteins, which can only be performed by the activated, GTP-bound form of Ras protein. The inactive, GDP-form of Ras protein shows significantly weaker binding to Raf kinase (Herrmann et al., 1995). Complex formation is not linked to stimulation of GTPase activity of Ras protein, and thus it is assumed that termination of signal transduction only occins on dissociation of Raf kinase or hydrolysis of the boimd GTP. The lifetime of the GTP state is determined by the GTPase activity of the Ras protein, which itself is subject to regulation by GAP proteins. 

The structural element involved in complex formation on the c-Rafl kinase side is the Ras binding domain, foimd in the CRl domain. On the Ras protein side, the L2 and L4 loops of the effector region are involved in complex formation. Both structural elements of Ras protein imdergo a distinct reorientation on transition from the inactive GDP form to the activated GTP state of Ras protein, and are thus very suitable as signal elements. 

Insight into the molecular interaction between Raf kinase and Ras protein was obtained from the crystal structure of the complex between the Ras binding domain of c-Rafl kinase and a Ras-like protein, RaplA protein (Nassar et al., 1995, review Wit-tinghofer and Nassar, 1996). 

The structure of the complex indicates a specific interaction between P-sheet structures of Raf kinase and structural elements of RaplA protein belonging to the switch I region, which are thus part of the RaplA protein effector domain. Since RaplA protein has a very similar structure to Ras protein, it is assumed that Ras protein also interacts with Raf kinase via its switch I region. 

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