TARGETING OF BCL-2 TO THE MITOCHONDRIAL OUTER-MEMBRANE BY A COOH-TERMINAL SIGNAL ANCHOR SEQUENCE

The protooncogene product Bcl-2 is an integral membrane protein that f unctions as a suppressor of programmed cell death. It contains a singl e predicted transmembrane segment located at its COOH terminus. Here, we show that the transmembrane domain of human Bcl-2 functions as a mi tochondrial signal anchor sequence that targets and inserts the protei n into the outer membrane in an N(cyto)-C(in). orientation, leaving th e bulk of the polypeptide facing the cytosol. Deletion of the COOH-ter minal 22 amino acids of Bcl-2 abrogated protein targeting, whereas fus ion of this domain to the COOH terminus of dihydrofolate reductase res ulted in targeting and insertion of the hybrid protein into the outer membrane in a manner similar to that of Bcl-2. The sequence of the hyd rophobic core of the Bcl-2 signal anchor is similar to the correspondi ng region of the NH2-terminal signal anchor of the mitochondrial outer membrane protein in yeast, Mas70p. A synthetic peptide comprising the Mas70p signal anchor sequence effectively competed for insertion of B cl-2 into the outer membrane but had no effect on the comparatively lo w association that Bcl-2 makes with endoplasmic reticulum microsomes. Insertion of Bcl-2 into the mitochondrial outer membrane is mechanisti cally different than its association with microsomes.