Samantha Stam

Isoforms Confer Characteristic Force Generation and Mechanosensation by Myosin II Filaments.

Myosin II isoforms with varying mechanochemistry and filament size interact with filamentous actin (F-actin) arrays to generate contractile forces in muscle and nonmuscle cells. How myosin II force production is shaped by isoform-specific motor properties and environmental stiffness remains poorly understood. Here, we used computer simulations to analyze force production by an ensemble of myosin motors against an elastically tethered actin filament. We found that force output depends on two timescales: the duration of F-actin attachment, which varies sharply with the ensemble size, motor duty ratio, and external load; and the time to build force, which scales with the ensemble stall force, gliding speed, and environmental stiffness. Although force-dependent kinetics were not required to sense changes in stiffness, the myosin catch bond produced positive feedback between the attachment time and force to trigger switch-like transitions from transient attachments, generating small forces, to high-force-generating runs. Using parameters representative of skeletal muscle myosin, nonmuscle myosin IIB, and nonmuscle myosin IIA revealed three distinct regimes of behavior, respectively: 1) large assemblies of fast, low-duty ratio motors rapidly build stable forces over a large range of environmental stiffness; 2) ensembles of slow, high-duty ratio motors serve as high-affinity cross-links with force buildup times that exceed physiological timescales; and 3) small assemblies of low-duty ratio motors operating at intermediate speeds are poised to respond sharply to changes in mechanical context-at low force or stiffness, they serve as low-affinity cross-links, but they can transition to force production via the positive-feedback mechanism described above. Together, these results reveal how myosin isoform properties may be tuned to produce force and respond to mechanical cues in their environment.

Filament rigidity and connectivity tune the deformation modes of active biopolymer networks

Significance Living cells spontaneously change their shape in physiological processes like cell migration and division. Forces generated by molecular motors on biopolymers must underlie these dynamics, but how molecular-scale forces give rise to cellular-scale shape changes is unknown. We use experimental measurements on reconstituted actomyosin networks and computer simulations to show that polymer stiffness and connectivity regulate motor-generated stresses and, in turn, longer-length-scale shape deformations. Importantly, we find that filament rigidity controls whether stresses transmitted are uniaxial or biaxial and that, for rigid filaments, the connectivity can control a transition between extensile and contractile deformations. These results have implications for how conserved molecular mechanisms give rise to diverse morphogenic events in cells. Molecular motors embedded within collections of actin and microtubule filaments underlie the dynamics of cytoskeletal assemblies. Understanding the physics of such motor-filament materials is critical to developing a physical model of the cytoskeleton and designing biomimetic active materials. Here, we demonstrate through experiments and simulations that the rigidity and connectivity of filaments in active biopolymer networks regulates the anisotropy and the length scale of the underlying deformations, yielding materials with variable contractility. We find that semiflexible filaments can be compressed and bent by motor stresses, yielding materials that undergo predominantly biaxial deformations. By contrast, rigid filament bundles slide without bending under motor stress, yielding materials that undergo predominantly uniaxial deformations. Networks dominated by biaxial deformations are robustly contractile over a wide range of connectivities, while networks dominated by uniaxial deformations can be tuned from extensile to contractile through cross-linking. These results identify physical parameters that control the forces generated within motor-filament arrays and provide insight into the self-organization and mechanics of cytoskeletal assemblies.

Cutting through the Noise: The Mechanics of Intracellular Transport

Intracellular transport of organelles and proteins is driven by multiple ATP-dependent processes. Recently in Cell, Guo et al. (2014) developed a technique, force-spectrum microscopy, to measure intracellular forces and demonstrate that large motion of cellular components can be produced by random ATP-dependent fluctuations within the cytoplasm.