Factors that modify the molecular size of phospholamban, the 23,000-dalton cardiac sarcoplasmic reticulum phosphoprotein.

Phospholamban, a 23,000-dalton phosphoprotein present in cardiac muscle sarcoplasmic reticulum vesicles is quantitatively dissociated into three smaller sized phosphorylated components of 11,000, 15,000, and 20,000 daltons when sarcoplasmic reticulum, solubilized in 1% (w/v) sodium dodecyl sulfate and 1 mM MgCl2, is heated at 71 degrees C or greater for 2 min. This dissociation is inhibited by Mg2+ (50% at approximately 5 mM). The 23,000-dalton phosphoprotein reformed from the 11,000-, 15,000-, and 20,000-dalton phosphorylated components when phosphorylated sarcoplasmic reticulum that had been boiled in 1% sodium dodecyl sulfate and 1 MM MgCl2 was stored for 1 week at -70 degrees C. We propose that the 23,000-dalton phosphorylated protein is a trimer, composed of three subunits with molecular mass of 11,000, 8,000, and 4,000 daltons. In this model, only the 11,000-dalton subunit would be phosphorylated. The partial dissociation of the 23,000-dalton phosphorylated protein would result in the formation of the 19,000 (11,000 + 8,000)-dalton or the 15,000 (11,000 + 4,000)- dalton phosphorylated components. Full dissociation of the 23,000-dalton phosphorylated protein would result in the formation of the 11,000-dalton phosphorylated component.