The trans-membrane domain of Bcl-2α, but not its hydrophobic cleft, is a critical determinant for efficient IP3 receptor inhibition

Abstract

The anti-apoptotic Bcl-2 protein is emerging as an efficient inhibitor of IP₃R function, contributing to its oncogenic properties. Yet, the underlying molecular mechanisms remain not fully understood. Using mutations or pharmacological inhibition to antagonize Bcl-2’s hydrophobic cleft, we excluded this functional domain as responsible for Bcl-2-mediated IP₃Rs inhibition. In contrast, the deletion of the C-terminus, containing the trans-membrane domain, which is only present in Bcl-2α, but not in Bcl-2β, led to impaired inhibition of IP₃R-mediated Ca²⁺ release and staurosporine-induced apoptosis. Strikingly, the trans-membrane domain was sufficient for IP₃R binding and inhibition. We therefore propose a novel model, in which the Bcl-2’s C-terminus serves as a functional anchor, which beyond mere ER-membrane targeting, underlies efficient IP₃R inhibition by (i) positioning the BH4 domain in the close proximity of its binding site on IP₃R, thus facilitating their interaction; (ii) inhibiting IP₃R-channel openings through a direct interaction with the C-terminal region of the channel downstream of the channel-pore. Finally, since the hydrophobic cleft of Bcl-2 was not involved in IP₃R suppression, our findings indicate that ABT-199 does not interfere with IP₃R regulation by Bcl-2 and its mechanism of action as a cell-death therapeutic in cancer cells likely does not involve Ca²⁺ signaling.