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A novel Bax-associating protein, named MAP-1 (Modulator of Apoptosis), has been identified in a yeast two-hybrid screen. MAP-1 contains a BH3-like (BH:Bcl-2 homology) motif and mediates caspase-dependent apoptosis in mammalian cells when overexpressed. MAP-1 homodimerizes and associates with the proapoptotic Bax and the prosurvival Bcl-2 and Bcl-X_L of the Bcl-2 family in vitro and in vivo in mammalian cells. Mutagenesis analyses revealed that the BH3-like domain in MAP-1 is not required for its association with Bcl-X_L but is required for association with Bax and for mediating apoptosis. Interestingly, in contrast to other Bax-associating proteins such as Bcl-X_Land Bid, which require the BH3 and BH1 domains of Bax, respectively, for binding, the binding of MAP-1 to Bax appears to require all three BH domains (BH1, BH2, and BH3) of Bax, because point mutation of the critical amino acid in any one of …