Proteolytic Studies on the Transduction Mechanism of Sarcoplasmic Reticulum Ca2+-ATPase
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 Ca2+ transport is blocked while partial reactions like Ca2+ binding, ATP phosphorylation, and Ca2+-occlusion are left intact. However, formation of the so-called E2P state (either from the phosphorylated species formed in the presence of ATP and Ca2+ or from the Ca2+-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 Ca2+ 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.
transduction ca2+-atpase proteinase k na+ k+-atpase proton atpases