Identification of core structural residues in the sequentially diverse and structurally homologous Bcl-2 family of proteins.

The Bcl-2 family of proteins regulates the intrinsic pathway of apoptosis and plays a significant role in mitochondrial outer membrane permeabilization. Bcl-2 homologues belonging to both anti- and pro-apoptotic classes have been identified in diverse organisms. While anti-apoptotic Bcl-2 proteins possess up to four BH sequence domains (BH1-BH4), the pro-apoptotic counterparts have either three BH (BH1-BH3) domains or only the BH3 domain. Many anti-apoptotic viral homologues do not seem to have any detectable BH homology regions and exhibit a very low level of sequence identity with other Bcl-2 family members. However, structures determined for several Bcl-2 anti- and pro-apoptotic proteins and their viral homologues show a remarkably conserved helical fold characterized by a central hydrophobic helix surrounded by five or six amphipathic helices. In this study, we have analyzed 16 nonredundant Bcl-2 structures from human, mouse, Caenorhabditis elegans, and five different viral species. While the length of the central hydrophobic helix is preserved in all the Bcl-2 structures, variations in length are observed for other helices. We performed multiple-structure alignment of all 16 structures. Eighty structurally equivalent positions, the bulk of them in the helical regions, constituted the ungapped blocks in the structure-based sequence alignment. Analysis of helix bundle geometry indicates that helix-helix packing differed in different Bcl-2 structures. This is presumably to accommodate disparate residue substitutions. Residue properties such as solvent accessibility, conservation of chemical nature, and/or size and involvement in interhelical interactions were analyzed in each position of the ungapped alignment regions. A sequence motif made up of small amino acids has been detected in the central helix that is proposed to be important for helix-helix association. We have found that residues in 22 positions in the helical regions are buried, exhibit conservation in hydrophobicity and/or size, and participate in interhelical interactions in at least 12 of the 16 structures studied. We also found 15 additional positions in which residues exhibit two of the three properties investigated. We suggest that these positions constitute the important structural core in the diverse Bcl-2 family members and could play a significant role in the folding of the protein. Results of our studies have been used in the identification of three putative Bcl-2 homologues from three different viral organisms. This study will help in the genome-wide identification of hitherto unrecognized Bcl-2 family members, especially in viral genomes.