Crystal structures of the calcium pump and sarcolipin in the Mg2+-bound E1 state

The X-ray crystal structures of SERCA1a, a Ca 2+ -ATPase from the sarcoplasmic reticulum, in the presence and absence of sarcolipin are reported; the structures indicate that sarcolipin stabilizes SERCA1a in an ‘open’ state that has not been well characterised previously, in which SERCA1a has not yet accepted calcium into its two high-affinity binding sites. How calcium drives muscle cells Muscle cell contraction and relaxation are controlled by the rise and fall of cytosolic calcium concentrations, initiated by the release of Ca 2+ from the sarcoplasmic reticulum (SR) and terminated by its re-sequestration by the SR Ca 2+ -ATPase (SERCA). Two papers in this issue of Nature present the X-ray crystal structures of SERCA in the presence of sarcolipin, a small membrane protein that regulates SERCA in skeletal muscle. The structures indicate that sarcolipin traps SERCA in a previously unknown 'open' state, in which SERCA has not yet accepted calcium into its two high-affinity binding sites. P-type ATPases are ATP-powered ion pumps that establish ion concentration gradients across biological membranes, and are distinct from other ATPases in that the reaction cycle includes an autophosphorylation step. The best studied is Ca 2+ -ATPase from muscle sarcoplasmic reticulum (SERCA1a), a Ca 2+ pump that relaxes muscle cells after contraction, and crystal structures have been determined for most of the reaction intermediates 1 , 2 . An important outstanding structure is that of the E1 intermediate, which has empty high-affinity Ca 2+ -binding sites ready to accept new cytosolic Ca 2+ . In the absence of Ca 2+ and at pH 7 or higher, the ATPase is predominantly in E1, not in E2 (low affinity for Ca 2+ ) 3 , and if millimolar Mg 2+ is present, one Mg 2+ is expected to occupy one of the Ca 2+ -binding sites with a millimolar dissociation constant 4 , 5 . This Mg 2+ accelerates the reaction cycle 4 , not permitting phosphorylation without Ca 2+ binding. Here we describe the crystal structure of native SERCA1a (from rabbit) in this E1·Mg 2+ state at 3.0 Å resolution in addition to crystal structures of SERCA1a in E2 free from exogenous inhibitors, and address the structural basis of the activation signal for phosphoryl transfer. Unexpectedly, sarcolipin 6 , a small regulatory membrane protein of Ca 2+ -ATPase 7 , is bound, stabilizing the E1·Mg 2+ state. Sarcolipin is a close homologue of phospholamban, which is a critical mediator of β-adrenergic signal in Ca 2+ regulation