RELATION BETWEEN CROSSBRIDGE STRUCTURE AND ACTOMYOSIN ATPASE ACTIVITYIN RAT-HEART

Cardiac myofilaments contain proteins that regulate the interaction be tween actin and myosin. In the thick filament, there are several prote ins that may contribute to the regulation of the contraction. The myos in binding protein C, or C protein, has 4 sites that can be phosphoryl ated by a Ca2+-calmodulin-controlled kinase, protein kinase A or prote in kinase C. Using electron microscopy and optical diffraction, we exa mined the structure of thick filaments isolated from rat ventricles wi th either the alpha or beta isoform of myosin heavy chain (MHC) and th e effect of specific phosphorylation of C protein on the structure. In thick filaments with alpha-MHC, crossbridges were clearly visible. Ph osphorylation of C protein by protein kinase A extended the crossbridg es from the backbone of the filament, changed their orientation, incre ased the degree of order of the crossbridges, and decreased the flexib ility of the crossbridges. Crossbridges in filaments with beta-MHC wer e less ordered and apparently more flexible. Phosphorylation of C prot ein in beta-MHC-containing filaments did not extend the crossbridges a nd did not alter degree of order or flexibility. The relative flexibil ity of the crossbridges inferred from the optical diffraction pattern correlated well with the rate of ATP hydrolysis by actomyosin. These r esults suggest that (1) crossbridge flexibility is an important parame ter in setting the rate of crossbridge cycling, and (2) C protein-medi ated control of the position and flexibility of crossbridges may regul ate actomyosin ATPase activity by modifying the kinetics of crossbridg e cycling.