Figure 3.

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].