ALTERATION OF MYOSIN CROSS-BRIDGES BY PHOSPHORYLATION OF MYOSIN-BINDING PROTEIN-C IN CARDIAC-MUSCLE

In addition to the contractile proteins actin and myosin, contractile filaments of striated muscle contain other proteins that are important for regulating the structure and the interaction of the two force-gen erating proteins, In the thin filaments, troponin and tropomyosin form a Ca-sensitive trigger that activates normal contraction when intrace llular Ca is elevated. In the thick filament, there are several myosin -binding proteins whose functions are unclear. Among these is the myos in-binding protein C (MBP-C), The cardiac isoform contains four phosph orylation sites under the control of cAMP and calmodulin-regulated kin ases, whereas the skeletal isoform contains only one such site, sugges ting that phosphorylation in cardiac muscle has a specific regulatory function, We isolated natural thick filaments from cardiac muscle and, using electron microscopy and optical diffraction, determined the eff ect of phosphorylation of MBP-C on cross bridges. The thickness of the filaments that had been treated with protein kinase A was increased w here cross bridges were present, No change occurred in the central bar e zone that is devoid of cross bridges, The intensity of the reflectio ns along the 43-nm layer line, which is primarily due to the helical a rray of cross bridges, was increased, and the distance of the first pe ak reflection from the meridian along the 43-nm layer line was decreas ed, The results indicate that phosphorylation of MBP-C (i) extends the cross bridges from the backbone of the filament and (ii) increases th eir degree of order and/or alters their orientation, These changes cou ld alter rate constants for attachment to and detachment from the thin filament and thereby modify force production in activated cardiac mus cle.