Characterization and optimization of the in vitro motility assay for fundamental studies of myosin II

Sammanfattning: Myosin II is the molecular motor responsible for muscle contraction. It transforms the chemical energy in ATP into mechanical work while interacting with actin filaments in so called cross-bridge cycles. Myosin II or its proteolytic fragments e.g., heavy meromyosin (HMM) can be adsorbed to moderately hydrophobic surfaces in vitro, while maintaining their ability to translocate actin filaments. This enables observation of myosin-induced actin filament sliding in a microscope. This “in vitro motility assay” (IVMA) is readily used in fundamental studies of actomyosin, including studies of muscle contraction. The degree of correlation of the myosin II function in the IVMA with its function in muscle depends on how the myosin molecules are arranged on the surface. Therefore a multi-technique approach, including total internal reflection spectroscopy, fluorescence interference contrast microscopy and quartz crystal microbalance with dissipation, was applied to characterize the HMM surface configurations. Several configurations with varying distributions were identified depending on the surface property. The most favorable HMM configurations for actin binding were observed on moderately hydrophobic surfaces. The effects on actomyosin function of different cargo sizes and amount of cargo loaded on an actin filament were also investigated. No difference in sliding velocities could be observed, independent of cargo size indicating that diffusional processive runs of myosin II along an actin filament are not crucial for actomyosin function in muscle. Furthermore, a tool for accurate velocity measurements appropriate for IVMAs at low [MgATP] was developed by utilizing the actin filament capping protein CapZ. These improvements allowed an investigation of the [MgATP]-velocity relationship to study possible processivity in fast skeletal muscle myosin II.  It is shown that the [MgATP]–velocity relationship is well described by a Michaelis-Menten hyperbola.  In addition, statistical cross-bridge modeling showed that the experimental results are in good agreement with recent findings of actomyosin cross-bridge properties, e.g., non-linear cross-bridge elasticity. However, no effect of inter-head cooperativity could be observed. In conclusion, the described results have contributed to in-depth understanding of the actomyosin cross-bridge cycle in muscle contraction.