Motor Mobility Determines Actin-Myosin Network Contraction Characteristics

Dynamic behaviours driven by intracellular cytoskeleton such as migration, division and endocytosis are mainly regulated by the contraction of cytoskeleton network consisting of cytoskeleton and motor proteins. Depending on the functional demand in each cell function, various structures and contraction characteristics of cytoskeleton networks are observed. For example, in the early phase of cytokinesis, chromosome segregation is progressed by the radial-directional contraction of the linear assembly of microtubules and motor protein dynein. During cleavage furrow ingression, the ring-shaped actin-myosin structure attached to the inner surface of cellular membrane shrinks at a constant speed by the circumferential-directional contraction of the ring.<br/>Many have studied how the contractile behaviours of cytoskeleton-motor protein networks depend on protein concentrations, chemical environments and geometric boundaries. In vitro experiments found that concentrations of motor protein and network crosslinkers play a crucial role in modulating contraction speed of the cytoskeleton-motor network. KCl concentration is critical for the global and synchronized contraction of actin-myosin network. The speed and length scale of the network contraction were significantly affected by the network shape such as square and circle. Nonetheless, it remains unclear how the mechanical boundary condition of the motor protein determines the dynamics characteristics of cytoskeleton-motor protein network contraction.<br/>Here, we analysed the contractile behaviours of actin-myosin network in two different mechanical boundary conditions of motor protein, myosin. We prepared “immobilized motor” (IMM) condition by firmly attaching myosin thick filaments to a glass surface. In “mobile motor” (MM) condition, actin network cross-linked with actin binding proteins (ABPs) were prepared on a glass surface coated with cellular lipid membrane allowing movement of motor proteins. Contraction of actin-myosin network in IMM and MM conditions were visualized using fluorescence-conjugated actin. We found that the relative concentration of actin and ABP which defines a network connectivity determines the contraction shape and speed. While contracting actin-myosin networks exhibited radial patterns in IMM condition, a global and isotropic contraction was observed in MM condition. Contraction speed increased with the network connectivity in MM but decreased with it in IMM condition.<br/>To elucidate the mechanism of the actin contraction depending on the boundary condition of motor protein, we developed a computational model tracking each component of network such as actin, ABP and myosin during contraction. The radial contractile pattern of actin network was exhibited in IMM condition similar to the experimental results. The computational simulation found that local actin-myosin clusters gathered into a larger one leading to a global contraction in MM condition.<br/>In summary, we showed how the characteristics of actin-myosin network contraction depend on the mechanical boundary condition of myosin motor protein. Changes of contraction behaviours in terms of contraction pattern and speed by the network connectivity depends on the mobility condition of motor protein. While the contraction speed increased proportional to network connectivity in MM, it decreased in IMM condition. Our study provides an intuition into not only the mechanism of cell dynamics but also the design of biomimetic systems. By modulating the mechanical boundary condition of motor protein, a dynamic molecular system transporting substances to a target destination can be designed.