The Bcl-2 Protein Family Member Bok Binds to the Coupling Domain of Inositol 1,4,5-Trisphosphate Receptors and Protects Them from Proteolytic Cleavage

Bok is a member of the Bcl-2 protein family that controls intrinsic apoptosis. Bok is most closely related to the pro-apoptotic proteins Bak and Bax, but in contrast to Bak and Bax, very little is known about its cellular role. Here we report that Bok binds strongly and constitutively to inositol 1,4,5-trisphosphate receptors (IP 3 Rs), proteins that form tetrameric calcium channels in the endoplasmic reticulum (ER) membrane and govern the release of ER calcium stores. Bok binds most strongly to IP 3 R1 and IP 3 R2, and barely to IP 3 R3, and essentially all cellular Bok is IP 3 R bound in cells that express substantial amounts of IP 3 Rs. Binding to IP 3 Rs appears to be mediated by the putative BH4 domain of Bok and the docking site localizes to a small region within the coupling domain of IP 3 Rs (amino acids 1895–1903 of IP 3 R1) that is adjacent to numerous regulatory sites, including sites for proteolysis. With regard to the possible role of Bok-IP 3 R binding, the following was observed: (i) Bok does not appear to control the ability of IP 3 Rs to release ER calcium stores, (ii) Bok regulates IP 3 R expression, (iii) persistent activation of inositol 1,4,5-trisphosphate-dependent cell signaling causes Bok degradation by the ubiquitin-proteasome pathway, in a manner that parallels IP 3 R degradation, and (iv) Bok protects IP 3 Rs from proteolysis, either by chymotrypsin in vitro or by caspase-3 in vivo during apoptosis. Overall, these data show that Bok binds strongly and constitutively to IP 3 Rs and that the most significant consequence of this binding appears to be protection of IP 3 Rs from proteolysis. Thus, Bok may govern IP 3 R cleavage and activity during apoptosis. Background: Bok is a Bcl-2 protein family member, with largely unknown properties. Results: Bok binds constitutively to IP 3 receptors and protects them from proteolysis, but does not appear to alter their Ca 2+ channel activity. Conclusion: Bok is novel component of IP 3 receptor complexes. Significance: A new locus for studies on IP 3 receptor and apoptosis regulation has been defined.