Model for NM2 filament assembly in cells. (A) Cytosolic myosin monomers in the actin network; large pore size in actin “net.” (B–D) Three non-mutually exclusive mechanisms for nascent filament assembly. (B) RLC kinases drive conversion from folded 10S to unfolded assembly-competent 6S monomers. (C) Cytosolic myosin monomer concentration increases from NM2 filament turnover in other areas of the cell. (D) Retraction collapses actin network, increases monomer density, and potentiates NM2 filament assembly. (E) Local monomers add to nascent NM2 filament until it becomes a mature filament. (F) Established NM2 filament associates with local monomers and nascent filaments, potentiating assembly. Sub-resolution NM2 filament stack/cluster continues to grow. (G–I) Sub-resolution NM2 filament stack/cluster partitions. NM2 filaments bound to actin filaments moving apart in a parallel manner will remain in register to build a sarcomeric filament stack, while NM2 filaments bound to actin filaments moving apart in a disorganized manner will generate disordered filament clusters. (J) Separated, discrete myosin stacks and clusters continue to grow to build higher-order networks. (K) NM2 filaments disassemble, releasing monomers that are redistributed throughout the cell to feed RLC kinases (B) and elevate cytosolic monomer levels (C).