Abstract
Sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA) and phospholamban (PLB) are essential for intracellular Ca(2+) transport in myocytes. Ca(2+)-dependent activation of SERCA-PLB provides a control function that regulates cytosolic and SR Ca(2+) levels. Although experimental and computational studies alone have led to a greater insight into SERCA-PLB regulation, the structural mechanisms for Ca(2+) binding reversing inhibition of the complex remain poorly understood. Therefore, we have performed atomistic simulations totaling 32.7 μs and cell-based intramolecular fluorescence resonance energy transfer (FRET) experiments to determine structural changes of PLB-bound SERCA in response to binding of a single Ca(2+) ion. Complementary MD simulations and FRET experiments showed that open-to-closed transitions in the structure of the headpiece underlie PLB inhibition of SERCA, and binding of a single Ca(2+) ion is sufficient to shift the protein population toward a structurally closed structure of the complex. Closure is accompanied by functional interactions between the N-domain β5-β6 loop and the A-domain and the displacement of the catalytic phosphorylation domain toward a competent structure. We propose that reversal of SERCA-PLB inhibition is achieved by stringing together its controlling modules (A-domain and loop Nβ5-β6) with catalytic elements (P-domain) to regulate function during intracellular Ca(2+) signaling. We conclude that binding of a single Ca(2+) is a critical mediator of allosteric signaling that dictates structural changes and motions that relieve SERCA inhibition by PLB. Understanding allosteric regulation is of paramount importance to guide therapeutic modulation of SERCA and other evolutionarily related ion-motive ATPases.