Crystal structure of a vertebrate smooth muscle myosin motor domain and its complex with the essential light chain: Visualization of the pre-power stroke state

The crystal structures of an expressed vertebrate smooth muscle myosin motor domain (MD) and a motor domain-essential light chain (ELC) complex (MDE), both with a transition state analog (MgADP . AlF4-) in the active site, have been determined to 2.9 Angstrom and 3.5 Angstrom resolution, respectively. The MDE structure with an ATP analog (MgADP . BeFx) was also determined to 3.6 Angstrom resolution. In all three structures, a domain of the C-terminal region, the "converter," is rotated similar to 70 degrees from that in nucleotide-free skeletal subfragment 1 (S1). We have found that the MDE-BeFx and MDE-AlFx structures are almost identical, consistent with the fact that they both bind weakly to actin. A comparison of the lever arm positions in MDE-AlF4- and in nucleotide-free skeletal S1 shows that a potential displacement of similar to 10 nm can be achieved during the power stroke.