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 AIF(4)(-) in the absence of Ca2+, which correspond to the E2P ground state and E2 similar to P transition state, respectively. The luminal gate is open with BeF3- and closed with AIF4-. These and the El similar to P center dot 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 MI-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 aspartyl phosphate to closing of the gate.