Dual function of Ras in Raf activation

The small guanine nucleotide binding protein p21(Ras) plays an important ro le in the activation of the Raf kinase. However, the precise mechanism by w hich Raf is activated remains unclear. It has been proposed that the sole f unction of p21(Ras) in Raf activation is to recruit Raf to the plasma membr ane. We have used Drosophila embryos to examine the mechanism of Raf (Draf) activation in the complete absence of p21(Ras) (Ras1). We demonstrate that the role of Ras1, in Draf activation is not limited to the translocation o f Draf to the membrane through a Ras1-Draf association. In addition, Rasl i s essential for the activation of an additional factor which in turn activa tes Draf.