After proteinase K-induced excision of five amino acid residues in the semiconserved polypeptide chain linking the end of the A domain with the S3/M3 transmembrane segment we find that Ca(2+) transport is blocked while partial reactions like Ca(2+) binding, ATP phosphorylation, and Ca(2+)-occlusion are left intact. However, formation of the so-called E2P state (either from the phosphorylated species formed in the presence of ATP and Ca(2+) or from the Ca(2+)-depleted unphosphorylated species) is blocked. We conclude that the proteinase K-treated ATPase, while maintaining many of the partial reactions, is incapable of energy transduction because of the absence of an E2P state with Ca(2+) binding sites exposed to the intravesicular space. Sequence comparisons and mutagenesis data point to an important role in energy transduction of P-type ATPases of a conserved motif located at the end of the A domain.