Turnover of myofibrillar proteins in cultured muscle cells from normal and dystrophic chick embryos.

Rates of protein degradation and synthesis were determined in myofibrillar and non-myofibrillar fractions and in myofibrillar subunits of cultured muscle cells from normal and dystrophic chick embryos of genetically closely related lines. Growth characteristics of normal and dystrophic cells were identical as measured by DNA, RNA and protein accumulation. Degradation rates of myofibrillar and non-myofibrillar protein determined from label-chase experiments were the same in normal and dystrophic cultures. In similar experiments in which degradation rates of eight different components of the myofibrillar fraction were measured, a spectrum of degradation rates was obtained indicating that myofibril components turn over individually or in groups, rather than as an intact unit. Myosin heavy chain was the slowest turning over component of those measured, while actin was among the most rapid. No differences were found between normals and dystrophics in the turnover of any of the components. Fractional rates of protein synthesis were also determined for the myofibrillar and non-myofibrillar fractions of cultured cells. The rates obtained with normal and dystrophic cells were indistinguishable and were the same for the myofibrillar and non-myofibrillar fractions. Initial amino acid incorporation rates into 17 separate components of the myofibrillar fractions were compared in normal and dystrophic cells. No significant differences were found.