Influences of protein kinase A and D-cAMP on actin-myosin interaction and energy consumption of cardiac muscles

Cardiac myosin binding protein C

Myosin binding protein C (MyBP-C) is one of a group of myosin binding proteins that are present in the myofibrils of all striated muscle. The protein is found at 43-nm repeats along 7 to 9 transverse lines in a portion of the A band where crossbridges are found (C zone). MyBP-C contains myosin and titin binding sites at the C terminus of the molecule in all 3 of the isoforms (slow skeletal, fast skeletal, and cardiac). The cardiac isoform also includes a series of residues that contain 3 phosphorylatable sites and an additional immunoglobulin module at the N terminus that are not present in skeletal isoforms. The following 2 major functions of MyBP-C have been suggested: (1) a role in the formation of the sarcomeric myofibril as a result of binding to myosin and titin and (2) in the case of the cardiac isoform, regulation of contraction through phosphorylation. The first is supported by the demonstrated effect of MyBP-C on the packing of myosin in the thick filament, the coincidence of appearance of sarcomeres and MyBP-C during myofibrillogenesis, and the defective formation of sarcomeres when the titin and/or myosin binding sites of MyBP-C are missing. The second is supported by the specific phosphorylation sites in cardiac MyBP-C, the presence in the thick filament of an enzyme specific for MyBP-C phosphorylation, the alteration of thick filament structure by MyBP-C phosphorylation, and the accompaniment of MyBP-C phosphorylation with all major physiological mechanisms of modulation of inotropy in the heart.

Relation between crossbridge structure and actomyosin ATPase activity in rat heart

Cardiac myofilaments contain proteins that regulate the interaction between actin and myosin. In the thick filament, there are several proteins that may contribute to the regulation of the contraction. The myosin binding protein C, or C protein, has 4 sites that can be phosphorylated by a Ca2+-calmodulin-controlled kinase, protein kinase A or protein kinase C. Using electron microscopy and optical diffraction, we examined the structure of thick filaments isolated from rat ventricles with either the alpha or beta isoform of myosin heavy chain (MHC) and the effect of specific phosphorylation of C protein on the structure. In thick filaments with alpha-MHC, crossbridges were clearly visible. Phosphorylation of C protein by protein kinase A extended the crossbridges from the backbone of the filament, changed their orientation, increased the degree of order of the crossbridges, and decreased the flexibility of the crossbridges. Crossbridges in filaments with beta-MHC were less ordered and apparently more flexible. Phosphorylation of C protein in beta-MHC-containing filaments did not extend the crossbridges and did not alter degree of order or flexibility. The relative flexibility of the crossbridges inferred from the optical diffraction pattern correlated well with the rate of ATP hydrolysis by actomyosin. These results suggest that (1) crossbridge flexibility is an important parameter in setting the rate of crossbridge cycling, and (2) C protein-mediated control of the position and flexibility of crossbridges may regulate actomyosin ATPase activity by modifying the kinetics of crossbridge cycling.