Proposed model for myosin filament formation in vitro. Unphosphorylated myosin II exists in folded monomeric form. It can assemble to form antiparallel folded dimers and then antiparallel folded tetramers. RLC phosphorylation (RLC-p) promotes unfolding. Antiparallel straight dimers and tetramers assemble to form filaments. Bipolar filaments are formed in striated muscle and side-polar filaments are formed in smooth muscle. Adapted from Dasbiswas et al. (2018), Liu et al. (2017), and Milton et al. (2011).