Near-Atomic Structure of the 10S form of Myosin II: Implications for Inhibition, Activation and Disease

Abstract

Myosin II is the molecular motor that powers contractility in muscle and nonmuscle cells. The two-headed molecule can exist in two conformations: compact (10S sedimentation coefficient) and extended (6S). The extended conformation assembles into filaments, which pull on actin to generate muscle contraction and cell motility. In the 10S structure, the tail is folded into three segments and the heads bend back and interact with each other and the tail, switching off ATPase activity, actin-activation and filament assembly. This energy-conserving, storage form can be activated by phosphorylation of its regulatory light chains (RLC) to form functional filaments as needed. We have solved the structure of smooth muscle 10S myosin to 4.3 Å resolution by cryo-EM, revealing near-atomic insights into its structure and inhibitory function (Yang et al., Nature, in press). The resulting refined atomic model shows clear secondary structure (α-helices, coiled-coil), with side-chain detail in some regions. The motor domains have the same (ADP.Pi) conformation, but their regulatory domains differ, allowing the two heads to meet asymmetrically. The coiled-coil shows considerable variations in pitch along its length. The regulatory domain backbone remains mostly helical as it joins the tail, with little apparent melting. The model reveals that the heads are prevented from binding to actin by steric clashes involving the tail and the other head, while their ATPase activity is impeded by constraints on movement of their converter domains. The positively charged N-terminal region of the RLC interacts with negative charge on the tail near the RLC phosphorylation site, suggesting a simple mechanism for activation. Disease-causing mutations in smooth and nonmuscle myosin II occur close to sites of head interaction with the tail, suggesting that interference with the 10S structure may underlie such diseases.