The trans-membrane domain of Bcl-2α, but not its hydrophobic cleft, is a critical determinant for efficient IP(3) receptor inhibition

The anti-apoptotic Bcl-2 protein is emerging as an efficient inhibitor of IP(3)R 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 IP(3)Rs 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 IP(3)R-mediated Ca(2+) release and staurosporine-induced apoptosis. Strikingly, the trans-membrane domain was sufficient for IP(3)R 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 IP(3)R inhibition by (i) positioning the BH4 domain in the close proximity of its binding site on IP(3)R, thus facilitating their interaction; (ii) inhibiting IP(3)R-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 IP(3)R suppression, our findings indicate that ABT-199 does not interfere with IP(3)R regulation by Bcl-2 and its mechanism of action as a cell-death therapeutic in cancer cells likely does not involve Ca(2+) signaling.