Crystallographic Basis for Calcium Regulation of Sodium Channels

Abstract

Voltage-gated sodium channels underlie the rapid regenerative upstroke of action potentials and are modulated by cytoplasmic calcium ions through a poorly understood mechanism. We describe the 1.35 A crystal structure of Ca2+ bound calmodulin (Ca2+/CaM) in complex with the inactivation gate (DIII-IV linker) of the cardiac sodium channel (NaV1.5). In conjunction with isothermal titration calorimetry (ITC), we identify novel inactivation gate mutations that enhance or diminish Ca2+/CaM binding, which, in turn, sensitize or abolish Ca2+ regulation of full-length channels in electrophysiological experiments. Additional ITC experiments support a model whereby a single Ca2+/CaM bridges the C-terminus IQ motif to the DIII-IV linker via individual N- and C-lobes. The data suggest that Ca2+/CaM impedes binding of the inactivation gate to its receptor, thus biasing inactivation toward more depolarized potentials.