EVIDENCE FOR ALPHA-HELICAL CONFORMATION OF AN ESSENTIAL N-TERMINAL REGION IN THE HUMAN BCL2 PROTEIN

A region occupying approximately 24 amino acids near the N terminus of human Bcl2 is essential for this cytoplasmic membrane protein's abili ty to inhibit apoptosis. Systematic mutagenesis of this N-terminal reg ion indicates that only five hydrophobic and aromatic residues within it are specifically required for function. Computerized secondary stru cture prediction, together with circular dichroism spectroscopy of syn thetic peptides, indicates that the region encompassing these five res idues has the propensity to take on an alpha-helical conformation in t he presence of SDS micelles, which presumably mimic the hydrophobic su rfaces of cellular membranes or polypeptides. The five critical residu es are predicted to be clustered on one face of this putative helix, w here they might serve to mediate protein protein contacts involved in the multimerization of Bcl2 or in the interaction of Bcl2 with other, as yet unidentified components of the apoptotic pathway. Apparent stru ctural homologues of this helical motif are also present in at least s ome other anti-apoptotic proteins from the Bcl2 family but not in thos e family members that tend to potentiate, rather than inhibit, apoptos is.