Calmodulin Dependent Protein Kinase II (CaMKII) Interacts with and Phosphorylates Cardiac Troponin and Tropomyosin

Abstract

Protein phosphorylation is a major regulatory mechanism for contractility and rate of relaxation in the heart. The cAMP-dependent protein kinase A (PKA) and the calmodulin-dependent protein kinase II (CaMKII) are well established modulators of cardiac protein phosphorylation. PKA and CaMKII phosphorylate a set of key regulatory proteins that control the excitation contraction coupling cycle. CaMKII phosphorylation of cardiac MyBP-C at Ser282 increases the number of myosin heads simultaneously attaching to actin, enhances the maximum force development and decreases myofibrillar calcium sensitivity accelerating relaxation. Immunoprecipitation of CaMKII delta (the predominant CaMKII form in heart) from murine cardiac tissues showed troponin subunits as potential interacting partners. Using mammalian two-hybrid system CaMKII delta was found to weakly interact with cardiac troponin I and troponin T and moderately with cardiac troponin C. CaMKII binds to and is activated by calmodulin (CaM). The tertiary structure of troponin C is similar to that of CaM and it may be possible that troponin C is binding CaMKII in a similar manner to how CaM binds CaMKII. CaMKII delta was also found to interact with alpha tropomyosin suggesting that CaMKII could readily bind several proteins on the thin filament. In vitro phosphorylation of recombinant cardiac troponin complexes showed that CaMKII readily phosphorylates cardiac troponin T with little phosphorylation of troponin I or C detected. Phosphorylation studies also showed that tropomyosin can also be phosphorylated by CaMKII. These results suggest that CaMKII can interact with several thin filament proteins but phosphorylates only a few of these proteins. Proteins which interact with but are not phosphorylated by CaMKII may serve as anchoring proteins allowing the CaMKII to phosphorylate other proteins nearby.