OLIGOMERIZATION ACTIVATES C-RAF-1 THROUGH A RAS-DEPENDENT MECHANISM

THE c-Raf-1 proto-oncoprotein is a Ras-GTP-regulated protein kinase(1) that associates in situ with 14-3-3 proteins(2,3), which are naturall y dimeric(4,5). In COS cells, recombinant Raf is found in oligomeric a ssemblies. To examine whether induced oligomerization can alter Raf ki nase activity, sequences encoding the FK506-binding protein FKBP12 wer e fused to the amino terminus of c-Raf-1, introducing a binding site f or FK506. Oligomerization of recombinant FKBP-Raf in situ, induced by the addition of the dimeric FK506 derivative FK1012A, activated Raf ki nase activity at least half as well as epidermal growth factor (EGF). As with EGF, activation of FKBP-Raf by FK1012A is entirely Ras-GTP dep endent. Thus oligomerization of Raf per se promotes Raf activation thr ough a Ras-dependent mechanism.