THE BINDING-PROPERTIES AND BIOLOGICAL-ACTIVITIES OF BCL-2 AND BAX IN CELLS EXPOSED TO APOPTOTIC STIMULI

The oncogene product Bcl-2 protects cells from apoptosis whereas its h omolog Bar functions to kill cells, Several binding partners of Bcl-2 and Bar have been isolated, but none of them has yet provided clues as to exactly how Bcl-2 and Bar work, According to one view, Bcl-2 and B ar interact with survival and death effector molecules, respectively, and neutralize each other through heterodimerization. Alternatively, B cl-2 requires Bar for death protection, and additional proteins bind t o the heterodimer to regulate its activity, Here we used a co-immunopr ecipitation strategy to distinguish between these two possibilities, W e show that the Bcl-2-Bax heterodimer is maintained, and no other prot ein associates stably in detectable amounts with Bcl-2, Bar, or the he terodimer in anti-Bcl-2 and anti-Bar immunoprecipitates from normal ce lls and cells exposed to apoptotic stimuli, Analysis of cells expressi ng various levels of Bcl-2 and Bar, however, revealed that the degree of protection against apoptosis does not correlate with the number of Bcl-2-Bax heterodimers but the amount of Bcl-2 that is free of Bar. In addition, the survival activity of Bcl-2 is unaffected when Bar expre ssion is ablated by an antisense strategy, Our findings suggest that t he Bcl-2-Bax heterodimer is a negative regulator of death protection, and that Bcl-2 requires neither Bar nor major, stable interactions wit h other cellular proteins to exert its survival function, We therefore propose that Bcl-2 acts as an enzyme (capturing substrates in a trans ient way), as a homodi- or multimer, or through the interaction with n on-proteaceous targets (lipids, ions).