Structure and regulation of the BsYetJ calcium channel in lipid nanodiscs

Abstract

BsYetJ is a bacterial homolog of transmembrane BAX inhibitor-1 motif-containing 6 (TMBIM6) membrane protein that plays a key role in the control of calcium homeostasis. However, the BsYetJ (or TMBIM6) structure embedded in a lipid bilayer is uncharacterized, let alone the molecular mechanism of the calcium transport activity. Herein, we report structures of BsYetJ in lipid nanodiscs identified by double electron–electron resonance spectroscopy. Our results reveal that BsYetJ in lipid nanodiscs is structurally different from those crystallized in detergents. We show that BsYetJ conformation is pH-sensitive in apo state (lacking calcium), whereas in a calcium-containing solution it is stuck in an intermediate, inert to pH changes. Only when the transmembrane calcium gradient is established can the calcium-release activity of holo-BsYetJ occur and be mediated by pH-dependent conformational changes, suggesting a dual gating mechanism. Conformational substates involved in the process and a key residue D171 relevant to the gating of calcium are identified. Our study suggests that BsYetJ/TMBIM6 is a pH-dependent, voltage-gated calcium channel.