Myosin binding protein C (MyBP-C) is essential for the formation of normal thick filaments and sarcomeres: The fact that the cardiac isoform of MyBP-C contains 3 phosphorylation sites which are not present in the skeletal isoform suggests that variation of the effect of MyBP-C on the structure of the thick filament by phosphorylation may be a mechanism for the modulation of the contraction in cardiac muscle. Almost every transmitter-based physiological mechanism for altering contractility is accompanied by a change in phosphorylation of MyBP-C in the thick filament. Work completed during the current period of support has provided the first direct evidence for change in thick filament and cross bridge structure from protein kinase A-mediated phosphorylation of MyBP-C. The changes in structure induced by the phosphorylation are consistent with the alterations in function that accompany the phosphorylation. The proposed hypothesis is that phosphorylation of MyBP-C in the heart is a mechanism for modulating contractility by modifying the structure of the thick filament and altering the interaction of myosin cross bridges with thin filaments. Our specific aim is to combine physiological, biochemical, structural and genetic studies of isolated rat cardiac muscle to determine the molecular mechanisms involved in this regulation. Functional and structural changes will be measured in parallel with changes in phosphorylation by combining electron microscopy and optical diffraction to detect changes in structure of isolated individual thick filaments, x-ray diffraction to detect changes in structure in the intact filament lattice and in intact cells, and isoelectric focussing and Western blotting to determine the specific pattern of phosphorylation of MyBP-C. The effect in isolated heart muscle of replacement of the whole MyBP-C molecule with genetically produced segments of the molecule will be studied to elucidate the basis for its influence on filament structure. The effect of genetic abnormalities in MyBP-C that have been produced in transgenic mice on the structure and function of the thick filament will also be examined. An understanding of this regulatory mechanism is important because of its potential for playing an important role in the normal function of the heart. Abnormalities in its structure and function are responsible for one of the more common forms of Familial Hypertrophic Cardiomyopathy.