HETEROLOGOUS EXPRESSION OF A CARDIOMYOPATHIC MYOSIN THAT IS DEFECTIVEIN ITS ACTIN INTERACTION

A point mutation in the heavy chain of cardiac myosin, resulting in re placement of an arginine (Arg) with glutamine (Gln), has been linked t o hypertrophic cardiomyopathy in humans (Geisterfer-Lowrance, A. A. T. , Kass, S., Tanigawa, G., Vosberg, H.-P., McKenna, W., Seidman, J. G., and Seidman, C. E. (1990) Cell 62, 999-1006). To determine the functi onal impact of this mutation, baculovirus-driven coexpression of myosi n heavy and light chains has been developed. The Arg-403 --> Gln mutat ion resulted in cardiac myosin with normal ATPase activity in the abse nce of actin. However, in the presence of actin, ATPase activity was g reatly reduced (V(max) decreased > 3.5-fold and K(app) increased >3-fo ld). In vitro motility was reduced nearly 5-fold by this single amino acid mutation. Thus, Arg-403 likely contributes to an important intera ction at the actin interface of myosin. Replacement, of Arg-403 with G ln leads to decreased rate(s) of transition within the actin-myosin cr ossbridge cycle. In humans, this mutation will result in decreased pow er output per unit area of cardiac muscle, likely providing a stimulus for hypertrophy.