Conformational and energetic basis of actomyosin force generation.

Abstract

Interactions between myosin and actin are essential in producing different types of cellular forces. Two key players in muscle contraction, myosin and actin, slide along each other to produce force using the energy from ATP hydrolysis. Targeting the motor domain of cardiac myosin, several small molecules have been proposed to treat cardiomyopathy. A clear mechanistic picture for the allosteric control in the actomyosin complex can facilitate computer-aided drug discovery. Atomistic simulation approaches such as molecular dynamics (MD) are limited for studying the motor functional cycle owing to the slow timescale as well as a lack of various intermediate structures. Here, integrating comparative modeling and enhanced sampling simulations, we reveal how the human cardiac myosin generates force in the cross-bridge cycle at atomic resolution. Initial conformational ensembles for different actomyosin states are learned from multiple structural templates with Rosetta. This enables us to efficiently sample the energy landscape of the system using Gaussian accelerated MD. Key myosin loop residues, whose substitutions are related to cardiomyopathy, are identified to form stable or transient interactions with actin. We find that the actin-binding cleft closure is allosterically coupled to the myosin core transitions and products release from the active site. Furthermore, a gate between switch I and II is suggested to control phosphate release at the pre-powerstroke state. The predicted metastable states and corresponding energetics are consistent with available experimental data and provide insights into the timing of elementary mechanochemical events.