Mutational analysis of the role of Glu309 in the sarcoplasmic reticulum Ca(2+)-ATPase of frog skeletal muscle.

Site-specific mutagenesis was used to analyse the role of the residue, Glu309, in the function of the Ca(2+)-ATPase of frog skeletal muscle sarcoplasmic reticulum by substitution with Ala or Lys. At pH 6.0, 100 microM Ca2+ was unable to prevent phosphorylation from Pi, consistent with previous observations on the Ca(2+)-ATPase of rabbit fast twitch muscle [Clarke, D.M., Loo, T.W, Inesi, G. and MacLennan, D.H. (1989) Nature 339, 476-478]. At neutral pH, however, micromolar concentrations of Ca2+ were sufficient to inhibit phosphorylation of the Glu309----Lys mutant from inorganic phosphate, suggesting that at least one high-affinity Ca2+ site was relatively intact in this mutant. The Glu309----Lys mutant was unable to form a phosphoenzyme from ATP at all Ca2+ concentrations studied (up to 12.5 mM), whereas phosphorylation of the Glu309----Ala mutant occurred at 12.5 mM Ca2+, but not at Ca2+ concentrations in the submillimolar range. Kinetic studies demonstrated a reduced rate of dephosphorylation of the E2P intermediate in the Glu309----Lys mutant. A less pronounced stabilization of E2P was observed with the Glu309----Ala mutant, suggesting a possible role of the charge at the position of Glu309 in phosphoenzyme hydrolysis.