MTD, A NOVEL BCL-2 FAMILY MEMBER ACTIVATES APOPTOSIS IN THE ABSENCE OF HETERODIMERIZATION WITH BCL-2 AND BCL-X-L

We have identified and characterized Mtd, a novel regulator of apoptos is. Sequence analysis revealed that Mtd is a member of the Bcl-2 famil y of proteins containing conserved BH1, BH2, BH3, and BH4 regions and a carboxyl-terminal hydrophobic domain. In adult tissues, Mtd mRNA was predominantly detected in the brain, liver, and lymphoid tissues, whi le in the embryo Mtd mRNA was detected in the liver, thymus, lung, and intestinal epithelium. Expression of Mtd promoted the death of primar y sensory neurons, 293T cells and HeLa cells, indicating that Mtd is a proapoptotic protein. Unlike all other known death agonists of the Bc l-2 family, Mtd did not bind significantly to the survival-promoting p roteins Bcl-2 or Bcl-X-L. Furthermore, apoptosis induced by Mtd was no t inhibited by Bcl-2 or Bcl-X-L. A Mtd mutant with glutamine substitut ions of highly conserved amino acids in the BH3 domain retained its ab ility to promote apoptosis, further indicating that Mtd does not promo te apoptosis by heterodimerizing with Bcl-2 or Bcl-X-L. Mtd-induced ap optosis was not blocked by broad range synthetic caspase inhibitors z- VAD-fmk or a viral protein CrmA. Mtd is the first example of a natural ly occurring Bcl-2 family member that can activate apoptosis independe ntly of heterodimerization with survival-promoting Bcl-2 and Bcl-X-L.