The structure of Boo/Diva reveals a divergent Bcl-2 protein

Abstract

Cells initiate apoptosis, a programmed cell suicide, in response to intracellular stress signals. Members of the B-cell lymphoma-2 (Bcl-2) family of proteins are critical to the regulation of the ‘‘intrinsic’’ or mitochondrial initiated cell death.1 This family comprises about 20 proteins in mammals and consists of members that either promote apoptosis the pro-apoptotic proteins or inhibit this action, the pro-survival proteins. Protein–protein interactions between the opposing factions mediate the life/death switch and it is thought the balance between the prosurvival and pro-apoptotic actions decides cell fate.2 Boo (Bcl-2 homolog of ovary, also known as Diva, Bcl-B, Bcl2L10, or Nrh) is a mouse Bcl-2 homolog, whose gene encodes a 191 residue 22 kDa protein that is highly expressed in the ovary,3 but only weakly in other tissues.4 Knockout of the boo gene does not give rise to an obvious phenotype suggesting a redundant function for this protein.4 There is little functional data available for Boo and initial reports on its function were contradictory, one indicating it had pro-survival activity,3 the other pro-apoptotic activity.5 Although the shared sequence identity between members of the Bcl-2 family is low, they harbor up to four regions of conserved sequence known as Bcl-2 homology (BH) domains, BH1–BH4.6 The pro-survival proteins bear 2–4 of these BH domains while the pro-apoptotic proteins are divided into those that bear a single BH domain, the BH3, and are known as the BH3-only proteins (members of this group include the proteins Bim, Bad, Bmf, Bid, Bik, Puma, Noxa and Hrk) and multidomain Bax-like proteins (Bax, Bak, and Bok) that share both sequence and structural homology to the pro-survival proteins Bcl-2, Bcl-xL, Bcl-w, Mcl-1, and A1.6 The presence of a BH3 domain is the minimal requirement for cell killing activity.7 The BH3-only proteins act as cellular sentinels that are activated in response to an apoptotic stimulus and bind in a hydrophobic groove located on the surface of the pro-survival proteins, neutralizing their action. This interaction frees the pro-apoptotic Baxlike proteins, which effect apoptosis by destroying the integrity of the mitochondrial outer membrane. Factors such as cytochrome c are consequently released from the mitochondrial intermembrane space, a process that results in the activation of caspases, cysteine aspartyl proteases, which execute a proteolytic cascade destroying the cell from within.1 The BH3 domain is a 13 residue sequence motif, F1sxxF2xxF3sDzF4B that contains a conserved leucine at position F2 and aspartic acid residue at position D.8 This short motif interacts with a hydrophobic groove on pro-survival proteins through the hydrophobic face of an amphipathic helix formed by the four hydrophobic residues, F1–F4. The other positions are: x is any residue,