Mutagenesis of segment 487Phe-Ser-Arg-Asp-Arg-Lys492 of sarcoplasmic reticulum Ca2+-ATPase produces pumps defective in ATP binding

Abstract

The lysine residue Lys492 located in the large cytoplasmic domain of sarcoplasmic reticulum Ca2+-ATPase is implicated in nucleotide binding through affinity labeling. The contribution of segment 487Phe-Ser-Arg- Asp-Arg-Lys492 to ATP binding and pump function has been investigated through the introduction of 11 site-directed amino acid mutations. ATP binding was measured through competitive inhibition of [γ-32P]2',3'- O(2,4,6-trinitrophenyl)-8-azido-adenosine triphosphate photolabeling of Lys492 or its substitute. Mutations F487S and positional swap F487S/S488F produced pumps that were severely defective in ATP binding (K(D) > I mM), and mutant F487S, together with F487E, exhibited low ATPase activity and low ATP- supported calcium transport and phosphorylation and failed to show CrATP- dependent Ca2+ occlusion. Mutations F487L, R489L, and K492Y were less inhibitory to ATP binding (K(D) = 8-49 μM) and, together with K492L and R489D/D490R, produced correspondingly smaller changes in ATP-mediated activities. The ATP dependence of ATPase activity of these five mutants showed deviations from the wild-type profile in the low, intermediate, and high concentration ranges, suggesting defects in ATP-dependent conformational changes. Mutations S488A and D490A had no effect on ATP binding (K(D) = 0.4 μM) or ATP-mediated activities. None of the mutations significantly affected phosphorylation from P(i) or acetyl phosphate-supported Ca2+ transport. Mutations F487L and F487S, and not those at residue 492, increased the K0.5 for Ca2+ activation of transport 2- and 8-fold, respectively. The results implicate Phe487, Arg489, and Lys492 in binding ATP in both a catalytic and a regulatory mode.