Cardiac Sarcalumenin: Phosphorylation, Comparison with the Skeletal Muscle Sarcalumenin and Modulation of Ryanodine Receptor

Abstract.: Cardiac sarcoplasmic reticulum (SR) contains an endogenous phosphorylation system that under specific conditions phosphorylates two proteins with apparent molecular masses of 150 and 130 kDa. The conditions for their phosphorylation are as for the skeletal muscle sarcalumenin and the histidine-rich Ca²⁺ binding protein (HCP) with respect to: (i) Ca²⁺ and high concentrations of NaF are required; (ii) phosphorylation is obtained with no added Mg²⁺ and shows a similar time course and ATP concentration dependence; (iii) inhibition by similar concentrations of La³⁺; (iv) phosphorylation is obtained with [γ-³²P]GTP; (v) ryanodine binding is inhibited parallel to the phosphorylation of the two proteins. The endogenous kinase is identified as casein kinase II (CK II) based on its ability to use GTP as effectively as ATP, and its inhibition by La³⁺. The association of CK II with the cardiac SR, even after EGTA extraction at alkaline pH, is demonstrated using antibodies against CK II. The cardiac 130 kDa protein is identified as sarcalumenin based on its partial amino acid sequence and its blue staining with Stains-All. Cardiac sarcalumenin is different from the skeletal muscle protein based on electrophoretic mobilities, immunological analysis, peptide and phosphopeptide maps, as well as amino acid sequencing. Preincubation of SR with NaF and ATP, but not with NaF and AMP-PNP caused strong inhibition of ryanodine binding. This is due to decrease in Ca²⁺- and ryanodine-binding affinities of the ryanodine receptor (RyR) by about 6.6 and 18-fold, respectively. These results suggest that cardiac sarcalumenin is an isoform of the skeletal muscle protein. An endogenous CK II can phosphorylate sarcalumenin, and in parallel to its phosphorylation the properties of the ryanodine receptor are modified.