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 x AIF4-) in the active site, have been determined to 2.9 A and 3.5 A resolution, respectively. The MDE structure with an ATP analog (MgADP x BeFx) was also determined to 3.6 A resolution. In all three structures, a domain of the C-terminal region, the "converter," is rotated approximately 70 degrees from that in nucleotide-free skeletal subfragment 1 (S1). We have found that the MDE-BeFx and MDE-AIF4- structures are almost identical, consistent with the fact that they both bind weakly to actin. A comparison of the lever arm positions in MDE-AIF4- and in nucleotide-free skeletal S1 shows that a potential displacement of approximately 10 nm can be achieved during the power stroke.