Translation of Ser¹⁶ and Thr¹⁷ phosphorylation of phospholamban into Ca²⁺-pump stimulation Available to Purchase

Stimulation of cardiac sarcoplasmic reticulum Ca²⁺-pump activity is achieved by phosphorylation of the oligomeric protein phospholamban at either Ser¹⁶ or Thr¹⁷. The altered mobility of phosphorylated forms of pentameric phospholamban has been utilized to demonstrate that the mechanisms of phosphorylation of the two sites differ. Phosphorylation of Ser¹⁶ by the AMP-dependent protein kinase proceeds via a random mechanism [Li, Wang and Colyer (1990) Biochemistry 29, 4535–4540], whereas phosphorylation of Thr¹⁷ by calmodulin-dependent protein kinase is shown here to proceed via a co-operative mechanism. This co-operative reaction mechanism was unaffected by the phosphorylation status of Ser¹⁶. These two mechanisms of phosphorylation generate very different phosphoprotein profiles depending on whether the Ser¹⁶ or Thr¹⁷ residue is phosphorylated. The translation of these patterns of phosphorylation into Ca²⁺-pump function was reviewed using a fluorimetric Ca²⁺-indicator dye, fluo-3, to measure Ca²⁺ uptake by cardiac sarcoplasmic reticulum vesicles. The rate of Ca²⁺ accumulation, which parallels Ca²⁺-pump activity, was stimulated in proportion with the stoichiometry of phospholamban phosphorylation, irrespective of whether phosphorylation was on Ser¹⁶ or Thr¹⁷.