MODULATION OF CELL-DEATH IN YEAST BY THE BCL-2 FAMILY OF PROTEINS

Bcl-2 family members are regulators of cell death. The precise biochem ical properties of these proteins are unclear although intrafamily pro tein protein association is thought to be involved. To elucidate struc ture-activity relationships among Bcl-2 proteins and identify the path ways in which they act, an inducible death suppressor assay was develo ped in yeast, Only Bar and Bak killed yeast via a process that did not require interleukin-1 beta converting enzyme-like proteases. Bax/Bak lethality was suppressed by coexpression of Bcl-2 family members that are anti-apoptotic in vertebrates, namely Bcl-xL, Bcl-2, Mcl-1, and A1 . Furthermore, Bcl-xL and Bcl-2 suppressed Bar toxicity by distinct me chanisms in yeast, Bad, Bcl-xS, and Ced-9 lacked suppressor activity. These inactive proteins bound to anti-apoptotic members of the Bcl-2 f amily but not to Bar or Bak. In contrast, most Bcl-2 family proteins t hat attenuated death bound to Bar and Bak, However, two mutants of Bcl -xL suppressed Bar-induced cell death while having no Bar binding acti vity. Therefore, Bcl-xL functions independently of Bar binding, perhap s by interacting with a common target or promoting a pathway that anta gonizes Bar, Thus, the pathways downstream of Bar and Bcl-xL may be co nserved between vertebrates and yeast. This suppressor assay could be used to isolate components of these pathways.