Mechanisms of activation/deactivation of the Raf-1 kinase. Identification of a mitogen-activated kinase which phosphorylates Raf on a negative regulatory residue

Cytosolic serinehreonine kinases convert extracellular stimuli into specific regulatory events affecting the pattern of gene expression, probably via phosphorylation of specific transcription factors. The Raf-1 kinase, in particular, is an important intermediate in the transduction of proliferative signals. During the past few years, substantial evidence supporting a direct role for Raf kinases in the development and maintenance of certain human cancers has been accumulating. Thus, Raf represents an attractive target for novel therapies aimed at interfering with its activation process and at eventually reversing the deregulated functions. We have recently shown that the deactivation of Raf-1 following mitogenic stimulation of this kinase correlates with the phosphorylation of a negative regulatory residue, Ser259. The specific aim of this project is to characterize the Ser259 kinase and to determine its relative importance as a negative regulator of Raf-1 activation in Ras-dependent and -independent systems. Ideally, the experiments proposed should lead to the identification of a negative regulator of Raf-1 activation and to the assessment of its potential as a therapeutic target.

The molecule at study, the Raf-1 kinase, is an important intermediate in the transduction of proliferative signals. During the past few years, substantial evidence supporting a direct role for Raf kinases in the development and maintenance of certain human cancers has been accumulating. Thus, Raf represents an attractive target for novel therapies aimed at interfering with its activation process. An essential step in Raf-1 activation is its translocation to the cell membrane. We have discovered that this step depends on the dephosphorylation of a specific serine residue, carried out by a phosphatase associated with Raf-1. Preventing Raf-1 dephosphorylation may represent an alternative strategy for controlling full-fledged activation in pathological situations.