Summary of steps involved in myocyte differentiation and myofibrillogenesis, with regards to actin dynamics. (a) Proliferating myoblasts are derived from determined myotome cells and possess an unspecialized actin cytoskeleton (grey lines). (b) As differentiation begins, these cells aggregate, characterized by the formation of localized stress fibers in a cortical actin wall (insert). Contractile function of these fibers is provided by nonmuscle myosin (NMM, arrow). (c) Myoblasts align themselves concurrent to substrate attachment and the elaboration of protocostameres (arrow). The stress-fiber-like cortical actin and NMM will form the premyofibril templates for subsequent myofibril assembly. (d) Fusion occurs, resulting in the formation of multinucleated myotubes. Myofibrils begin to form at the cell periphery, centered on costamere attachment points (arrow), constructed from premyofibril templates (insert). Premyofibrils consist of alternating bands of membrane-associated α-actinin (circles) and NMM (arrow). (e) As the myocyte matures, additional myofibrils will fill all available space, interconnected with one another and with organelles by desmin intermediate filaments. New myoblasts will continue to fuse to the terminal ends of the myotube to create a growing myofiber. Artwork by A. Pete.

Schematic diagram of the sarcomere and costamere protein complexes of striated muscle cells. Major components of the mature sarcomere and costamere are shown, along with the cytoskeletal and motor filament systems, in context with the sarcolemma and organelles of syncytial myocytes. Known chaperone or cochaperone molecules are shown in bold, along with their substrates. Arrows indicate regions where chaperone-mediated protein folding is essential to incorporate polymeric filament proteins. Modified from Sparrow and Schock [17], additional artwork by A. Pete.

Synthesis of the premyofibril model with the roles of nonmuscle myosin (NMM) in early differentiating myoblasts. Schematic representation of the molecular events leading from cytoskeletal actin to mature myofibrils. (a) Elaboration of the actin cytoskeleton in proliferating myoblasts leads to the formation of a cortical actin wall. (b) Stress-fiber-like structures in the cortical actin wall contain associated NMM-II, which allows for alignment and fusion of myoblasts. (c) Alignment and fusion are concurrent with early costamere formation, resulting in the anchorage of premyofibrils to the extracellular matrix. These sites serve as nucleation points, resulting in the formation of minisarcomeres with alternating bands of α-actinin and NMM. Incorporation of N-terminal titin occurs at this point as well. (d) Folding and lengthening of titin is concurrent with the organization of α-actinin into the Z-band and the incorporation of muscle MHC-II into the thick filament, displacing NMM and widening sarcomeres. (e) M-line proteins associate with MHC-II and C-terminal titin, creating the final banding pattern of mature myofibrils. Modified from Sparrow and Schock [17].

Proposed model of roles for Unc45b/nonmuscle myosin during early myofibrillogenesis. (a) Merged protein model of Unc-45b from the X-ray crystal structure of Drosophila UNC-45 and the solved NMR structure of the human Unc45a TPR domain (protein database ID 2DBA), showing the known active domains. Proposed functions of each section of the protein are indicated. Modified from Lee et al. [139]. (b) Flowchart of myofibrillogenesis, listing the stages where there is significant evidence to hypothesize the involvement of UNC-45. Events that likely involve UNC-45 chaperone function at each stage of myogenesis are noted, and probable cofactors for UNC-45 are indicated in red.