H36 is a species-specific, cell-surface antigen on differentiating newborn rat skeletal myoblasts and myogenic lines. This membrane antigen has been defined by a monoclonal antibody raised by the fusion of SP 2/0-Ag14 myeloma cells with spleen cells from mice immunized with myotubes derived from the myogenic E63 line. H36 antigen, isolated by immunoaffinity chromatography, is comprised of two polypeptides with apparent molecular weights of 98,000 and 117,000. Fluorescence photometry and radioimmunoassays have been used to follow quantitative and topographic changes in the H36 determinant during myogenesis. H36 is present at a basal level on replicating myoblasts; it increases on prefusion myoblasts and persists on myotubes. At or near the time of prefusion, it becomes concentrated between adjacent aligned myoblasts and localized on membrane "blebs". H36 is present on both skeletal and cardiac cells but absent from a variety of cells that include fibroblasts, neuronal cells, and smooth muscle. There are approximately 4 x 10(5) determinants per myoblast, and the Ka of the antibody is 3.8 x 10(8) liters/mol. The distributions of H36 on the top and attached surfaces of myoblasts and myotubes are distinct, which suggests localized specialization of these surfaces. H36 is an integral membrane component and upon cross-linking, it associates with the detergent-insoluble cytoskeletal framework. Inhibition of myogenesis by 5-bromodeoxyuridine or by calcium deprivation prevents the developmentally associated changes in the expression of H36. H36 is also absent or markedly reduced on the fu- and Ama102 developmentally defective mutant myoblast lines. We conclude that H36 is a muscle-specific, developmentally regulated cell-surface antigen that may have a role in myoblast differentiation and that can be used to determine the embryonic lineages of skeletal and cardiac muscle.
BAK1 Is Not a Target of the Pseudomonas syringae Effector AvrPto
