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

AbstractThe 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 Å and 3.5 Å resolution, respectively. The MDE structure with an ATP analog (MgADP·BeFx) was also determined to 3.6 Å resolution. In all three structures, a domain of the C-terminal region, the “converter,” is rotated ∼70° from that in nucleotide-free skeletal subfragment 1 (S1). We have found that the MDE–BeFx and MDE–AlF4− 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 ∼10 nm can be achieved during the power stroke