Mutation of the myosin converter domain alters cross-bridge elasticity

Abstract

Elastic distortion of a structural element of the actomyosin complex is fundamental to the ability of myosin to generate motile forces. An elastic element allows strain to develop within the actomyosin complex (cross-bridge) before movement. Relief of this strain then drives filament sliding, or more generally, movement of a cargo. Even with the known crystal structure of the myosin head, however, the structural element of the actomyosin complex in which elastic distortion occurs remained unclear. To assign functional relevance to various structural elements of the myosin head, e.g., to identify the elastic element within the cross-bridge, we studied mechanical properties of muscle fibers from patients with familial hypertrophic cardiomyopathy with point mutations in the head domain of the ␤-myosin heavy chain. We found that the Arg-719 3 Trp (Arg719Trp) mutation, which is located in the converter domain of the myosin head fragment, causes an increase in force generation and fiber stiffness under isometric conditions by 48 -59%. Under rigor and relaxing conditions, fiber stiffness was 45-47% higher than in control fibers. Yet, kinetics of active cross-bridge cycling were unchanged. These findings, especially the increase in fiber stiffness under rigor conditions, indicate that cross-bridges with the Arg719Trp mutation are more resistant to elastic distortion. The data presented here strongly suggest that the converter domain that forms the junction between the catalytic and the light-chain-binding domain of the myosin head is not only essential for elastic distortion of the cross-bridge, but that the main elastic distortion may even occur within the converter domain itself. It is widely accepted that active force and movement generated by muscle fibers result from structural changes in the head domain of the myosin molecule (the cross-bridge) while it is attached to the actin filament. These changes are thought to involve a tilting of the light-chain-binding domain of the myosin head relative to its catalytic domain (1-7). As a result of such structural changes distortion of an elastic element within the actin-attached myosin head allows strain to develop before movement (8). Relief of this strain drives sliding of actin filaments past the myosin filaments, or alternatively, if filament sliding is prevented, active force is generated because of continued strain of the elastic element. Despite the central significance of this concept, however, it remained unclear which part of the actomyosin complex represents the elastic element-i.e., the element that experiences the main elastic distortion while other parts act more like rigid bodies. Neither the known crystal structures of the myosin head nor cryo-electron microscopy with reconstruction of the actomyosin complex have resolved this question. Some authors considered elastic bending of the long, light-chain-binding ␣-helix an obvious candidate (4, 9, 10). The actin-myosin interface (11) or the junction between the lightchain domain and the catalytic domain of the...

Molecular mechanism regulating myosin and cardiac functions by ELC

“…Besides the lever arm [24], the converter domain was suggested to represent an alternative element of elastic distortion during force generation [25]. A FHC-related mutation within the converter domain substantially decreased myosin stiffness in fibers [26,27]. 3D-analysis of myosin-S1…”

Influence of Fluctuations in Actin Structure on Myosin V Step Size

“…While this assumption is justified for long lever arms, the effect of compliance in the converter domain could play an important role for short lever arms. Results on skeletal muscle myosin (with short lever arms) indeed show that at least half of the compliance is in the converter domain [17]. Therefore some care has to be taken when interpreting the results for 2IQ lever arms.…”

Optimal Flexibility for Conformational Transitions in Macromolecules

“…It is conceivable that this effect is exploited by evolution, for example, in the design of hinged molecular motors [6,7]. A promising candidate to test these ideas is myosin II, where mutations affecting the stiffness of the converter region and the activity of the motor are known [22,23]. Whether flexibility assisted barrier crossing is important in processive motors is difficult to test, since the rate of the conformational transition has to be considerably faster than the unbinding rate from the filament.…”