Integrating comparative modeling and accelerated simulations reveals conformational and energetic basis of actomyosin force generation

Abstract

Muscle contraction is performed by arrays of contractile proteins in the sarcomere. Serious heart diseases, such as cardiomyopathy, can often be results of mutations in myosin and actin. Direct characterization of how small changes in the myosin-actin complex impact its force production remains challenging. Molecular dynamics (MD) simulations, although capable of studying protein structurefunction relationships, are limited owing to the slow timescale of the myosin cycle as well as a lack of various intermediate structures for the actomyosin complex. Here, employing comparative modeling and enhanced sampling MD simulations, we show how the human cardiac myosin generates force during the mechanochemical cycle. Initial conformational ensembles for different myosin-actin 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 metastable interactions with the actin surface. We find that the actin-binding cleft closure is allosterically coupled to the myosin core transitions and ATP-hydrolysis product release from the active site. Furthermore, a gate between switch I and switch II is suggested to control phosphate release at the pre-powerstroke state. Our approach demonstrates the ability to link sequence and structural information to motor functions. Significance Statement: Interactions between myosin and actin are essential in producing various cellular forces. Targeting cardiac myosin, several small molecules have been developed to treat cardiomyopathy. A clear mechanistic picture for the allosteric control in the actomyosin complex can potentially facilitate drug design by uncovering functionally important intermediate states. Here, integrating Rosetta comparative modeling and accelerated molecular dynamics, we reveal how ATP-hydrolysis product release correlates with powerstroke and myosin tight binding to actin. The predicted metastable states and corresponding energetics complement available experimental data and provide insights into the timing of elementary mechanochemical events. Our method establishes a framework to characterize at an atomistic level how a molecular motor translocates along a filament.