Apoptosis regulation by interaction of Bcl-2 protein and Raf-1 kinase

The Bcl-2 protein is over-produced in many types of human tumors and suppresses apoptosis induced by a wide-variety of stimuli, including chemotherapeutic drugs and γ-irradiation. The biochemical mechanism of action of the Bcl-2 protein however remains enigmatic. Here we show that Bcl-2 can be co-immunoprecipitated with the serine/threonine-specific Raf-1 kinase both in a mammalian hemopoietic cell 32D.3 and when the two proteins are produced in Sf9 insect cells using recombinant baculoviruses. Though analysis of Raf-1 deletion mutants suggested that the C-terminal half of the protein which contains the catalytic domain is sufficient for co-immunoprecipitation with Bcl-2, Raf-1 does not appear to induce phosphorylation of Bcl-2 protein in 32D.3 and Sf9 cells. Furthermore, a mutant form of Raf-1 that lacks kinase activity could still be co-immunoprecipitated with Bcl-2 in Sf9 cells, suggesting that the interaction of these proteins does not reflect a kinase-substrate relation. Gene transfer experiments using 32D.3 hemopoietic cells demonstrated functional synergy between Bcl-2 and Raf-1 with regards to suppression of apoptosis induced by growth factor withdrawal. Taken together, these observations for the first time functionally link Bcl-2 to a signal transducing protein and suggest that the interaction of the Bcl-2 and Raf-1 proteins may be responsible for their ability to cooperate in the suppression of apoptosis.