The Structure of the Actin-Smooth Muscle MYOSIN II Complex in the Rigor State

Abstract

The actin-based motility of myosin utilizes catalysis of ATP to drive relative sliding of actin and myosin. Attachment to F-actin triggers conformational changes in the myosin head that accelerates release of catalytic products coupled to changes in actin affinity and motion of a structural domain called the lever arm. The earliest detailed model based on cryoEM and X-ray crystallography postulated that higher F-actin affinity and lever arm movement were coupled to closure of a feature of the myosin head dubbed the actin-binding cleft. Several studies since then using crystallography of myosin-V and cryoEM of myosin-I and -II have provided details of this model. Compared with non-muscle myosins, there are comparatively few catalytic intermediates available for myosin II. Details of the myosin II interaction with actin may differ from those for non-muscle myosins due in part to different lengths of important surface loops. Here we report on a 0.8 nm reconstruction of actin decorated with the smooth muscle myosin-II motor domain (MD) in the rigor state obtained by cryoEM and Iterative Helical Real Space Reconstruction. Quasiatomic models for both F-actin and the MD were obtained independently three times using molecular dynamics flexible fitting. SD3 and SD4 of F-actin are nearly identical while SD1 and SD2 show the most difference. The MD density is less well defined compared to the actin filament due to incomplete saturation. A significant density that could be assigned to loop1 is present. Loop2 and the myopathy loop show significant contacts with actin but these contacts appear variable between the three independent fittings. The comparison between nucleotide free acto-MD structure and the prepower-stroke crystal structure of the smooth muscle myosin MD suggests significant conformational changes occurred that may relate to the weak to strong transition.