The structure of the rigor complex and its implications for the power stroke

Abstract

Decorated actin provides a model system for studying the strong interaction between actin and myosin. Cryo–energy–filter electron microscopy has recently yielded a 14 Å resolution map of rabbit skeletal actin decorated with chicken skeletal S1. The crystal structure of the cross–bridge from skeletal chicken myosin could not be fitted into the three–dimensional electron microscope map without some deformation. However, a newly published structure of the nucleotide–free myosin V cross–bridge, which is apparently already in the strong binding form, can be fitted into the three–dimensional reconstruction without distortion. This supports the notion that nucleotide–free myosin V is an excellent model for strongly bound myosin and allows us to describe the actin–myosin interface. In myosin V the switch 2 element is closed although the lever arm is down (post–power stroke). Therefore, it appears likely that switch 2 does not open very much during the power stroke. The myosin V structure also differs from the chicken skeletal myosin structure in the nucleotide–binding site and the degree of bending of the backbone ß–sheet. These suggest a mechanism for the control of the power stroke by strong actin binding.