Bok is a pro-apoptotic Bcl-2 protein with restricted expression in reproductive tissues and heterodimerizes with selective anti-apoptotic Bcl-2 family members

Abstract

In the intracellular death program, heteroand homodimerization of different 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 killer (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 (Mcl-1, BHRF1, and Bf l-1) but not other (Bcl-2, Bcl-xL, and Bcl-w) anti-apoptotic members. This finding is in direct contrast to the ability of other pro-apoptotic members (Bax, Bak, and Bik) to interact with all of the anti-apoptotic proteins. In addition, negligible interaction was found between Bok and different pro-apoptotic members. In mammalian cells, overexpression of Bok induced apoptosis that was blocked by the baculoviralderived cysteine protease inhibitor P35. Cell killing induced 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 analysis 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 restricted tissue distribution and heterodimerization properties could facilitate elucidation of apoptosis mechanisms in reproductive tissues undergoing hormone-regulated cyclic cell turnover.

Bcl-xS and Bax induce different apoptotic pathways in PC12 cells

“…The need for caspase activity was also reported in the induction of apoptosis by transient overexpression of some other anti-apoptotic members of the Bcl-2 family of proteins, for example Bid (Wang et al, 1996), Bak and Bik (Orth and Dixit, 1997) and Bok (Hsu et al, 1997).…”

“…The anti-apoptotic members of the Bcl-2 family include Bcl-2, Bcl-x L , Bcl-W, Mcl-1, B¯1/ A1, E1B 19k, Epstein-Barr virus BHRF1, and Caenorhabditis elegans Ced-9. Pro-apoptotic members include Bax, Bak, Bad, Bik, Bid, Bim, Hrk, Bok/Mtd, Blk and Bcl-x S (for review see Hegde et al, 1998;Hsu et al, 1997;Kelekar and Thompson, 1998;O'Connor et al, 1998;Strasser et al, 1997 and references therein). The homology between members of the Bcl-2 family resides mainly within four conserved domains, designated Bcl-2 homology (BH) domains BH1, BH2, BH3 and BH4.…”

Proapoptotic multidomain Bcl-2/Bax-family proteins: mechanisms, physiological roles, and therapeutic opportunities

“…41,42 However, more should be done to investigate cell death regulation when these mice are stressed in various ways. Expression of the bok gene is limited to testis, ovary, and uterus in mice, 43 though expression of the human ortholog of this gene may be more widespread, including liver and kidney. 44 Genotoxic injury experiments using bokÀ/À mice did not reveal a requirement for bok for cell death in ovaries or other organs.…”

Intracellular localization of the BCL-2 family member BOK and functional implications

“…18,19 Bok expression was first reported to be restricted to reproductive tissues. 16 However, we have recently shown that BOK can be detected in most tissues, albeit in several at only relatively low levels. 20 The BOK protein was found to localize to the mitochondria, the ER as well as to the nucleus.…”

“…2,[10][11][12][13][14][15] BOK (BCL-2-related ovarian killer) was identified in a yeast two-hybrid screen using MCL-1 as bait and was shown to selectively interact with MCL-1 and BFL-1/A1 but not with BCL-2 or BCL-X L . 16,17 Among the BCL-2 family members, BOK shows the highest evolutionary conservation. 18,19 Bok expression was first reported to be restricted to reproductive tissues.…”