Site-directed mutagenesis studies of energy coupling in the sarcoplasmic reticulum Ca(2+)-ATPase

Site-directed mutagenesis studies identifying residues important to energy transduction in the sarcoplasmic reticulum Ca(2+)-ATPase are reviewed. Mutations blocking the crucial E1P to E2P transition are located in the small and the large cytoplasmic domains, in the stalk segment S4 linking transmembrane segment M4 with the catalytic site, as well as in transmembrane segments M4 and M8. Mutations that block the dephosphorylation of the E2P phosphoenzyme intermediate are located in transmembrane segments M4, M5, and M6, i.e., in the same domain as the Ca(2+)-binding sites. Removal of the sidechain of Tyr763 located at the boundary between transmembrane segment M5 and the corresponding stalk segment S5 linking M5 with the catalytic site leads to uncoupling of ATP hydrolysis from Ca2+ uptake. Uncoupling may be due to efflux through the Ca(2+)-ATPase of Ca2+ that has been transported, and may thus be caused by a defective gating process in the late part of the catalytic cycle. A nearby located residue Lys758 is also involved in energy coupling, since its substitution with Ile activates dephosphorylation at high pH and slows the E2 to E1 transition.