Calcium ions (Ca 2+ ) are prominent cell signaling effectors that regulate a wide variety of cellular processes. Among the different players in Ca 2+ homeostasis, primary active Ca 2+ transporters are responsible for keeping low basal Ca 2+ levels in the cytosol while establishing steep Ca 2+ gradients across intracellular membranes or the plasma membrane. This review summarizes our current knowledge on the three types of primary active Ca 2+ -ATPases: the sarco(endo)plasmic reticulum Ca 2+ -ATPase (SERCA) pumps, the secretory pathway Ca 2+ -ATPase (SPCA) isoforms, and the plasma membrane Ca 2+ -ATPase (PMCA) Ca 2+ -transporters. We first discuss the Ca 2+ transport mechanism of SERCA1a, which serves as a reference to describe the Ca 2+ transport of other Ca 2+ pumps. We further highlight the common and unique features of each isoform and review their structure-function relationship, expression pattern, regulatory mechanisms, and specific physiological roles. Finally, we discuss the increasing genetic and in vivo evidence that links the dysfunction of specific Ca 2+ -ATPase isoforms to a broad range of human pathologies, and highlight emerging therapeutic strategies that target Ca 2+ pumps.

Sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) transporters regulate calcium signaling by active calcium ion reuptake to internal stores. Structural transitions associated with transport have been characterized by x-ray crystallography, but critical intermediates involved in the accessibility switch across the membrane are missing. We combined time-resolved x-ray solution scattering (TR-XSS) experiments and molecular dynamics (MD) simulations for real-time tracking of concerted SERCA reaction cycle dynamics in the native membrane. The equilibrium [Ca2]E1 state before laser activation differed in the domain arrangement compared with crystal structures, and following laser-induced release of caged ATP, a 1.5-ms intermediate was formed that showed closure of the cytoplasmic domains typical of E1 states with bound Ca2+ and ATP. A subsequent 13-ms transient state showed a previously unresolved actuator (A) domain arrangement that exposed the ADP-binding site after phosphorylation. Hence, the obtained TR-XSS models determine the relative timing of so-far elusive domain rearrangements in a native environment.

The sarcoplasmic/endoplasmic reticulum Ca 2+ -ATPase 2a (SERCA2a) performs active reuptake of cytoplasmic Ca 2+ and is a major regulator of cardiac muscle contractility. Dysfunction or dysregulation of SERCA2a is associated with heart failure, while restoring its function is considered as a therapeutic strategy to restore cardiac performance. However, its structure has not yet been determined. Based on native, active protein purified from pig ventricular muscle, we present the first crystal structures of SERCA2a, determined in the CPA-stabilized E2ÀAlF À 4 form (3.3 Å) and the Ca 2+occluded [Ca 2 ]E1-AMPPCP form (4.0 Å). The structures are similar to the skeletal muscle isoform SERCA1a pointing to a conserved mechanism. We seek to explain the kinetic differences between SERCA1a and SERCA2a. We find that several isoform-specific residues are acceptor sites for post-translational modifications. In addition, molecular dynamics simulations predict that isoformspecific residues support distinct intramolecular interactions in SERCA2a and SERCA1a. Our experimental observations further indicate that isoform-specific intramolecular interactions are functionally relevant, and may explain the kinetic differences between SERCA2a and SERCA1a.

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