The effect of N-terminal acetylation on Ca(2+)-ATPase inhibition by phospholamban

The peptide Ac-MEKVQYLTRSAIRRASTIEMPQQAR (Ac-PLB(1-25)) representing residues 1-25 of phospholamban (PLB) inhibited the maximal activity of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum by about 53%, with a Kd value of 5 microM; the equivalent non-acetylated peptide PLB(1-25) had no effect. However, it was found that the non-acetylated peptide increased the effective Kd value for inhibition by Ac-PLB(1-25) consistent with competitive binding to the ATPase, with a Kd value of 8 microM for PLB(1-25). The non-acetylated peptide must therefore be able to bind to the ATPase, but in a conformation that does not lead to inhibition of the ATPase. The identity of the N-terminal residue is important in determining the strength of binding; replacement of the Met residue by Ile led to fourfold weaker binding, again with only binding of the acetylated peptide leading to inhibition of ATPase activity.