A song of heads and tails: myosin II conformational regulation and filament dynamics shape force generation in non-muscle cells

Fig. 1. Types of myosin 2 reservoirs and assemblies. a In muscle cells, MM2 adopts the super-relaxed state (SRX), in which the head domains may be interacting (left, IHM state), or not, with each other and the stem of the coiled coil (IHM). ATP consumption in this state is minimal. Upon activation, the SRX state evolves into a disordered, relaxed state, where the head domains are available to interact with the thin (actin filament). Basal ATP expenditure is higher than in SRX. Finally, heads engage with F-actin in the active state, where ATP expenditure is maximal. b SM2 and c NM2 appear in a folded, cytoplasmic conformation that does not enable growth into large filaments. Upon conformational extension (See Fig. 2), SM2 forms stable bipolar sheets (b), and NM2 forms bipolar filaments (c)

Fig. 2. A model of SM2/NM2 filament formation. According to Korn and co-workers (Liu et al., 2017), SM2/NM2 10S monomers may form folded dimers or tetramers in the absence of RLC phosphorylation (a, blue arrows). Upon RLC phosphorylation (black arrows), dimers or tetramers extend into bipolar filaments with 2 or 4 heads for half-filament (b). These can grow by incorporation of individual 10S monomers or dimers. c Larger order filaments are maintained in homeostasis through incorporation of 10S monomers/dimers assisted by non-contractile Myo18 (green), directed by F-actin filaments (red) and/or assisted by UNC45a chaperone (mauve). d Monomers can also be exchanged through the establishment of electrostatic interactions between the exposed coiled-coils of the filament and the folded coiled-coil of monomers, also assisted by UNC45a

Fig. 3. SM2 and NM2 structures in smooth muscle and non-muscle cells, respectively. a Smooth muscle cells display polar arrays of SM2 connected to the plasma membrane by F-actin filaments interacting with diffuse membrane plaques. Upon nervous stimulation (yellow growth cones), SM2 undergoes coordinated contraction in a Ca2.+-dependent manner. b In polarized non-muscle cells, NM2 arrays appear associated with dorsal stress fibers, dorsal arches, and ventral stress fibers. The absence of NM2-A impairs the occurrence of all these structures, whereas NM2-B deletion impairs only the polarity defined by ventral stress fibers. The distribution of NM2 paralogs is not even and has been extensively reviewed elsewhere (Garrido-Casado et al., 2021; Vicente-Manzanares et al., 2009).

Fig. 4. Sarcomere-like features of NM2-A in hematopoietic cells. Human monocyte-derived dendritic cells cultured for 7 days in the presence of IL-4 (10 ng/mL) and GM-CSF (20 ng/mL) were allowed to adhere to 20 µg/mL fibronectin for 30 min, fixed, and stained for NM2-A (green), F-actin (red), and focal adhesion protein vinculin (blue). Magenta represents the co-localization of F-actin and vinculin; scale bars = 10 µm. Yellow boxes are magnified on black and white panels (NM2-A only). Note the sarcomere-like arrangement of NM2-A stacks

Fig. 5. Mutational hotspots in MYH7 and MYH9 genes. a MYH7 hotspots for hypertrophic cardiomyopathy (HCM) are shown in pink, dilated cardiomyopathy (DCM) in green, and distal myopathies in orange and brown. Regions with higher probability are shown in pale colors, and clustered hotspots are shown in intense colors. b Representative mutations causing MYH9-RD in NM2-A. See ref. (Asensio-Juárez et al., 2020) for additional details