How processing of aspartylphosphate is coupled to lumenal gating of the ion pathway in the calcium pump

Ca²⁺-ATPase of skeletal muscle sarcoplasmic reticulum is the best-studied member of the P-type or E1/E2 type ion transporting ATPases. It has been crystallized in seven different states that cover nearly the entire reaction cycle. Here we describe the structure of this ATPase complexed with phosphate analogs BeF₃^- and AlF₄^- in the absence of Ca²⁺, which correspond to the E2P ground state and E2∼P transition state, respectively. The luminal gate is open with BeF₃^- and closed with AlF₄^-. These and the E1∼P.ADP analog crystal structures show that a two-step rotation of the cytoplasmic A-domain opens and closes the luminal gate through the movements of the M1-M4 transmembrane helices. There are several conformational switches coupled to the rotation, and the one in the cytoplasmic part of M2 has critical importance. In the second step of rotation, positioning of one water molecule couples the hydrolysis of aspartylphosphate to closing of the gate.