Phospholamban regulation of cardiac sarcoplasmic reticulum (Ca²⁺-Mg²⁺)-ATPase: Mechanism of regulation and site of monoclonal antibody interaction

Monoclonal antibodies raised against canine cardiac sarcoplasmic reticulum phospholamban were used to study the structure-function relationship between phospholamban and the sarcoplasmic reticulum (SR) (Ca²⁺-Mg²⁺)-ATPase (Suzuki, T., and Wang, J. H. (1986) J. Biol. Chem. 261, 7018-7023). Additional monoclonal antibodies are characterized further. When five of these monoclonal antibodies were assessed for their ability to affect SR Ca²⁺ uptake three of these antibodies had no effect on SR Ca²⁺ uptake, whereas the other two monoclonals were able to stimulate SR Ca²⁺ uptake to levels similar to those caused by phosphorylation of phospholamban at different calcium concentrations. Using synthetic peptides corresponding to various portions of phospholamban in a competitive binding assay, it was possible to map the epitope site of monoclonals which stimulate the (Ca²⁺-Mg²⁺)-ATPase activity to phospholamban residues 7-16. These results implicate phospholamban residues 7-16 in the regulation of the (Ca²⁺-Mg²⁺)-ATPase.