Coarse-grained simulations of actomyosin rings point to a nodeless model involving both unipolar and bipolar myosins

Figures

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  FIGURE 1:. Coarse-graining the actomyosin system: (A) Models of F-actin (green), myosins (tail in orange, heads in red), actin cross-linkers (pink), and membrane (yellow) (see the text for details). Note the same visualizations and colors are used for all following figures unless otherwise stated. (B) The ATPase cycle of myosin was modeled in five steps: myosin (1) binds ATP and releases actin, (2) hydrolyzes ATP, (3) binds actin, (4) releases phosphate, and (5) releases ADP.

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  FIGURE 2:. Setting up basic components of the simulated constriction system: F-actin (opposing polarities in green and cyan), bipolar myosin, cross-linkers, and membrane. Italic font indicates simulated times. (A) The initial ring was composed of F-actin, bipolar myosin, and membrane. (B) The actomyosin ring did not contract in the absence of cross-linkers. (C) In the presence of cross-linkers, the ring did contract. (D) Adding tethers between F-actin and the membrane caused the ring to constrict the membrane. (E) Adding regulatory factors (tension-dependent interaction between actin and myosin, filament orientation-dependent cross-linking, cofilin function, and actin turnover) reduced F-actin bending. (F) Histogram of straightness factors for filaments visualized by ECT (Swulius et al., 2018) (blue), in simulations without regulatory factors (red), and in simulations with regulatory factors (green).

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  FIGURE 3:. Exploration of different actomyosin models resulting from combining four actin configurations (A1–A4) with four myosin configurations (M1–M4). Snapshots of the 15 plausible resultant models are presented. Note that the combination of A4 and M4 is not plausible, since there are no tethers between the contractile ring and the membrane.

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  FIGURE 4:. The ring tension was calculated. Left, representative time courses of ring tensions in individual simulations of the 16 models; right, averages over five simulations for each model, with number 16 representing the final model. Error bars represent standard deviations.

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  FIGURE 5:. Representative features produced by the first 15 models. (A) Membrane puckering, as in Model 1, and large angles between filaments and membrane. (B) Tethering membrane-bound unipolar myosins in nodes or pairs, as in columns 1 and 2 of Figure 3, resulted in aggregation. (C) Individual unipolar myosins, as in Model 11, pulled filaments close to the membrane. (D) Bipolar myosins, as in Model 12, pulled filaments away from the membrane.

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  FIGURE 6:. Angles between actin filaments and the membrane calculated after 60 s of simulated time. Left, representative histograms of angles in individual simulations; right, averages over five simulations for each model with error bars representing standard deviation and the red horizontal line indicating the average angle (7.8°) measured from electron tomograms for a reference. The presence of puckers, as in Models 1, 4, and 13, causes filaments to form angles larger than those observed experimentally.

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  FIGURE 7:. The final model. (A) A zoomed-in view shows initial configuration of the ring, including untethered F-actin (green and cyan), membrane-attached unipolar myosins (red), and bipolar myosins (orange). During constriction, (B) membrane smoothness and circularity were preserved and distances between F-actin and the membrane as observed in tomograms were recapitulated (as shown in a zoomed-in view), and (C) the membrane-attached unipolar myosins (red) occupied the outer edge of the ring while the unattached bipolar myosins (orange) occupied the inner edge. (D) Average distance between the simulated F-actin and the membrane (arrow indicates the average distance measured in tomograms) and (E) constriction rate as a function of the unipolar myosin’s ATPase rate scaling factor, λ. Error bars represent standard deviations (n = 5).