Electrostatic and bending energies predict staggering and splaying in nonmuscle myosin II minifilaments

Abstract

Recent experiments with super-resolution live cell microscopy revealed that nonmuscle myosin II minifilaments are much more dynamic than formerly appreciated, often showing plastic processes such as splitting, concatenation and stacking. Here we combine sequence information, electrostatics and elasticity theory to demonstrate that the parallel staggers at 14.3, 43.2 and 72 nm have a strong tendency to splay their heads away from the minifilament, thus potentially initiating the diverse processes seen in live cells. In contrast, the straight antiparallel stagger with an overlap of 43 nm is very stable and likely initiates minifilament nucleation. Using stochastic dynamics in a newly defined energy landscape, we predict that the optimal parallel staggers between the myosin rods are obtained by a trial-and-error process in which two rods attach and re-attach at different staggers by rolling and zipping motion. The experimentally observed staggers emerge as the configurations with the largest contact times. We find that contact times increase from isoforms C to B to A, that A-B-heterodimers are surprisingly stable and that myosin 18A should incorporate into mixed filaments with a small stagger. Our findings suggest that nonmuscle myosin II minifilaments in the cell are first formed by isoform A and then convert to mixed A-B-filaments, as observed experimentally. Author summaryNonmuscle myosin II (NM2) is a non-processive molecular motor that assembles into minifilaments with a typical size of 300 nm to generate force and motion in the actin cytoskeleton. This process is essential for many cellular processes such as adhesion, migration, division and mechanosensing. Due to their small size below the resolution limit, minifilaments are a challenge for imaging with traditional light microscopy. With the advent of super-resolution microscopy, however, it has become apparent that the formation of NM2-minifilaments is much more dynamic than formerly appreciated. Modeling the electrostatic interaction between the rigid rods of the myosin monomers has confirmed the main staggers observed in experiments, but cannot explain these high dynamics. Here we complement electrostatics by elasticity theory and stochastic dynamics to show that the parallel staggers are likely to splay away from the main axis of the minifilament and that monomers attach and deattach with rolling and zipping motions. Based on the sequences of the different NM2-isoforms, we predict that isoform A forms the most stable homofilaments and that A-B-heterofilaments are also very stable.

Linking the Landscape of MYH9-Related Diseases to the Molecular Mechanisms that Control Non-Muscle Myosin II-A Function in Cells

“…Experimental data as well as modeling support the notion that NMII establishes electrostatic interactions to form parallel and anti-parallel arrays with well-defined periodicity. This is due to the amino acid sequence of specific regions within the coiled coil domain of NMII [73][74][75]. The current thinking is that, upon RLC phosphorylation, the NMII hexamer adopts a 6S conformation, which is highly unstable.…”

“…It is not precisely known why coiled coil mutations display less severe phenotypes. NMII-A dimers, similar to Drosophila melanogaster NMII [73], form parallel and anti-parallel arrays at different staggers [74]. This could potentially allow dimers to "skip" mutant regions by establishing electrostatic interactions that are less favorable in the case of the wild-type form, but more favorable in the case of the mutant ( Figure 3B).…”

“…. It is unlikely that bipolar filament formation (top) is largely affected as electrostatic interactions between coiled coils dominate this step [73,74]. Color codes of the different chains is as in Figure 1.…”

“…However, this remains to be determined experimentally. Cartoon depicts the integration of NMII-A, wild-type (magenta/purple) or mutant (black) into mini-filaments with other NMII paralogs, e.g., NMII-B (in orange/red tones), which is thermodynamically feasible, as shown in [74]. Color codes of the different chains is as in Figure 1.…”

“…Emerging evidence points to the ability of NMII-A to form heterodimers and co-polymerize with other NMII isoforms, particularly NMII-B and Myo18 [102][103][104]. A recent study suggests that A/B heterodimerization is favorable thermodynamically, opening the door for these heterodimers to have biological importance [74]. This is a clearly understudied part of the field regarding MYH9 mutations, as some mutations, particularly those affecting the coiled coil and NHT domains, may have a profound impact on the dynamics of heterodimerization.…”