Sarcolipin-Mediated Regulation of SERCA by Computer Simulations

Abstract

The sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) is a transmembrane pump that, upon ATP hydrolysis, pumps Ca2+ against a concentration gradient from the cytosol to the sarco- or endoplasmic reticulum, thus terminating muscle contraction and priming the cell for the next excitation-contraction stimulus1. SERCA function in skeletal muscle cells is regulated by sarcolipin2,3 (SLN), a 31 amino acid transmembrane peptide that inhibits SERCA by lowering its apparent Ca2+ affinity. By combining solid-state NMR and cross-linking experiments, we characterized the structure of the SERCA/SLN complex in its natural membrane environment in both Ca2-E1:ATP and Hn-E2:ATP states of the catalytic cycle. In this work, we employ molecular dynamics computer simulations to investigate SERCA's mechanism of regulation by mapping the effects of SLN binding on SERCA's free energy landscape, sub-microsecond structural dynamics, and allosteric coupling between the transmembrane and cytoplasmic domains. Our calculations indicate that upon binding to SERCA, SLN increases its average tilt angle with respect the unbound state. The interaction between the two proteins results in a shift of the free energy basin of SERCA and in an altered coupling between its transmembrane and cytoplasmic domains, which may be responsible for the reduced activity of SERCA in its SLN-bound state.