Critical roles of interdomain interactions for modulatory ATP binding to sarcoplasmic reticulum Ca2+-ATPase

The tilting motion of the central core reveals the transport mechanism of the sarco/endoplasmic reticulum Ca2+-ATPase

The sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) transports two Ca2+ ions per ATP hydrolyzed from the cytoplasm to the lumen. However, how the ATP hydrolysis remotely drives the Ca2+ transport is unclear. In the SERCA1a crystal structures, the ATP hydrolysis is accompanied by the notably increasing tilting angle of the central core (CC) and the Ca2+ transport, and the CC tilting angle dramatically decreases in the E2 to E1 transition. We demonstrated that the significantly increasing tilting motion of the CC drove the Ca2+ release in the molecular dynamics simulation of the R836A variant, and the dramatic spontaneous decrease in the CC tilting angle of the E2 state triggers the restart of the SERCA1a's transport cycle. The repulsion between the phosphorylated D351 and the phosphate groups in ADP triggers the release of ADP from the SERCA1a headpiece. We proposed a novel SERCA transport mechanism in which ATP hydrolysis drives a significant tilting motion of the CC, which drives Ca2+ transport and the A domain rotational motion in the E1P-ADP-2Ca2+ to E2P transition. The dramatic spontaneous decrease in the CC tilting angle of the E2 state drives the restart of the transport cycle.

Cryoelectron microscopy of Na+,K+-ATPase in the two E2P states with and without cardiotonic steroids

The E2P state of Na⁺,K⁺-ATPase, in which the ATPase is phosphorylated and of low affinity to Na⁺ with the extracellular gate opened, shows different biochemical properties depending on whether the phosphate is transferred from ATP in the forward reaction or from inorganic phosphate (P_i) in the backward reaction. We present here cryoelectron microscopy structures of Na⁺,K⁺-ATPase in the two E2P states, explaining their different biochemical properties established a half century ago. The new electron microscopy maps show previously unseen structural features, including unexpected binding modes of cardiotonic steroids, specific and medically important inhibitors of the ATPase, and stabilization by ATP of the E2P state.

Cryoelectron microscopy (cryo-EM) was applied to Na⁺,K⁺-ATPase (NKA) to determine the structures of two E2P states, one (E2P^ATP) formed by ATP and Mg²⁺ in the forward reaction, and the other (E2P^Pi) formed by inorganic phosphate (P_i) and Mg²⁺ in the backward reaction, with and without ouabain or istaroxime, representatives of classical and new-generation cardiotonic steroids (CTSs). These two E2P states exhibit different biochemical properties. In particular, K⁺-sensitive acceleration of the dephosphorylation reaction is not observed with E2P^Pi, attributed to the presence of a Mg²⁺ ion in the transmembrane cation binding sites. The cryo-EM structures of NKA demonstrate that the two E2P structures are nearly identical but Mg²⁺ in the transmembrane binding cavity is identified only in E2P^Pi, corroborating the idea that it should be denoted as E2P^Pi·Mg²⁺. We can now explain why the absence of transmembrane Mg²⁺ in E2P^ATP confers the K⁺ sensitivity in dephosphorylation. In addition, we show that ATP bridges the actuator (A) and nucleotide binding (N) domains, stabilizing the E2P^ATP state; CTS binding causes hardly any changes in the structure of NKA, both in E2P^ATP and E2P^Pi·Mg²⁺, indicating that the binding mechanism is conformational selection; and istaroxime binds to NKA, extending its aminoalkyloxime group deep into the cation binding site. This orientation is upside down compared to that of classical CTSs with respect to the steroid ring. Notably, mobile parts of NKA are resolved substantially better in the electron microscopy (EM) maps than in previous X-ray structures, including sugars sticking out from the β-subunit and many phospholipid molecules.

Structural dynamics of P-type ATPase ion pumps

P-type ATPases transport ions across biological membranes against concentration gradients and are essential for all cells. They use the energy from ATP hydrolysis to propel large intramolecular movements, which drive vectorial transport of ions. Tight coordination of the motions of the pump is required to couple the two spatially distant processes of ion binding and ATP hydrolysis. Here, we review our current understanding of the structural dynamics of P-type ATPases, focusing primarily on Ca2+ pumps. We integrate different types of information that report on structural dynamics, primarily time-resolved fluorescence experiments including single-molecule Förster resonance energy transfer and molecular dynamics simulations, and interpret them in the framework provided by the numerous crystal structures of sarco/endoplasmic reticulum Ca2+-ATPase. We discuss the challenges in characterizing the dynamics of membrane pumps, and the likely impact of new technologies on the field.

Determination of the ATP Affinity of the Sarcoplasmic Reticulum Ca(2+)-ATPase by Competitive Inhibition of [γ-(32)P]TNP-8N3-ATP Photolabeling

The photoactivation of aryl azides is commonly employed as a means to covalently attach cross-linking and labeling reagents to proteins, facilitated by the high reactivity of the resultant aryl nitrenes with amino groups present in the protein side chains. We have developed a simple and reliable assay for the determination of the ATP binding affinity of native or recombinant sarcoplasmic reticulum Ca(2+)-ATPase, taking advantage of the specific photolabeling of Lys(492) in the Ca(2+)-ATPase by [γ-(32)P]2',3'-O-(2,4,6-trinitrophenyl)-8-azido-adenosine 5'-triphosphate ([γ-(32)P]TNP-8N3-ATP) and the competitive inhibition by ATP of the photolabeling reaction. The method allows determination of the ATP affinity of Ca(2+)-ATPase mutants expressed in mammalian cell culture in amounts too minute for conventional equilibrium binding studies. Here, we describe the synthesis and purification of the [γ-(32)P]TNP-8N3-ATP photolabel, as well as its application in ATP affinity measurements.