BOK IS A PRO-APOPTOTIC BCL-2 PROTEIN WITH RESTRICTED EXPRESSION IN REPRODUCTIVE TISSUES AND HETERODIMERIZES WITH SELECTIVE ANTI-APOPTOTIC BCL-2 FAMILY MEMBERS

In the intracellular death program, hetero- and homodimerization of di fferent anti-and pro-apoptotic Bcl-2-related proteins are critical in the determination of cell fate, From a rat ovarian fusion cDNA library , we isolated a new pro-apoptotic Bcl-2 gene, Bcl-2-related ovarian ki ller (Bok), Bok had conserved Bcl-2 homology (BH) domains 1, 2, and 3 and a C-terminal transmembrane region present in other Bcl-2 proteins, but lacked the BH4 domain found only in anti-apoptotic Bcl-2 proteins , In the yeast two-hybrid system, Bok interacted strongly with some (M cl-1, BHRF1, and Bfl-1) but not other (Bcl-2, Bcl-xL, and Bcl-w) anti- apoptotic members, This finding is in direct contrast to the ability o f other pro-apoptotic members (Bax, Bak, and Bik) to interact with all of the anti-apoptotic proteins, In addition, negligible interaction w as found between Bok and different pro-apoptotic members, In mammalian cells, overexpression of Bok induced apoptosis that was blocked by th e baculoviral-derived cysteine protease inhibitor P35, Cell killing in duced by Bok was also suppressed following coexpression with Mcl-1 and BHRF1 but not with Bcl-2, further indicating that Bok heterodimerized only with selective anti-apoptotic Bcl-2 proteins, Northern blot anal ysis indicated that Bok was highly expressed in the ovary, testis and uterus, In situ hybridization analysis localized Bok mRNA in granulosa cells, the cell type that underwent apoptosis during follicle atresia , Identification of Bok as a new pro-apoptotic Bcl-2 protein with rest ricted tissue distribution and heterodimerization properties could fac ilitate elucidation of apoptosis mechanisms in reproductive tissues un dergoing hormone-regulated cyclic cell turnover.