IDENTIFICATION OF A NOVEL REGULATORY DOMAIN IN BCL-X(L) AND BCL-2

Bcl-x(L), a member of the Bcl-2 family, can inhibit many forms of prog ramed cell death. The three-dimensional structure of Bcl-x(L) identifi ed a 60 amino acid loop lacking defined structure, Although amino acid sequence within this region is not conserved among Bcl-2 family membe rs, structural modeling suggested that Bcl-2 also contains a large uns tructured region. Compared,vith the full-length protein, loop deletion mutants of Bcl-x(L) and Bcl-2 displayed an enhanced ability to inhibi t apoptosis. Despite enhanced function, the deletion mutants did not h ave significant alterations in the ability to bind pro-apoptotic prote ins such as Bar. The loop deletion mutant of Bcl-2 also displayed a qu alitative difference in its ability to inhibit apoptosis. Full-length Bcl-2 was unable to prevent anti-IgM-induced cell death of the immatur e B cell line WEHI-231. In contrast, the Bcl-2 deletion mutant protect ed WEHI-231 cells from death. Substantial differences were observed in the ability of WEHI-231 cells to phosphorylate the deletion mutant of Bcl-2 compared with full-length Bcl-2. Bcl-2 phosphorylation was foun d to be dependent on the presence of an intact loop domain, These resu lts suggest that the loop domain in Bcl-x(L) and Bcl-2 can suppress th e anti-apoptotic function of these genes and may be a target for regul atory post-translational modifications.