DIFFERENTIAL ASSEMBLY OF CYTOSKELETAL AND SARCOMERIC ACTINS IN DEVELOPING SKELETAL-MUSCLE CELLS IN-VITRO

Monoclonal antibodies (McAb) to actin were prepared to analyze the ass embly of actin isoforms in developing muscle cells in vitro. One of th e antibodies (SkA-06) was specific for a-sarcomeric actin isoforms in skeletal and cardiac muscles, while the others recognized cytoskeletal (beta, gamma) actin isoforms in smooth muscle and non-muscle tissues as well as the sarcomeric (alpha) actins. Using SkA-06 and a polyclona l antibody (PcAb) specific for cytoskeletal actins, the subcellular lo calization of the actin isoforms was examined by immunocytochemical me thods. While in developing young myotubes, cytoskeletal and sarcomeric actins were co-localized in nascent myofibrils or stress-fiber-like s tructures, sarcomeric actins predominated in striated myofibrils in mo re developed myotubes. When FITC-labeled cytoskeletal and sarcomeric a ctins were introduced into young myotubes by a microinjection method, the latter became detectable in striated structures sooner than the fo rmer but they were finally incorporated into striated myofibrils. Thes e results suggest that alpha-actin(s) as well as beta- and gamma-actin s can be incorporated into myofibrils, but alpha-actin(s) is assembled preferentially into myofibrils in developing muscle cells.