Analysis of conformational changes at the unique loop adjacent to the ATP binding site of smooth muscle myosin using...

Recent crystallographic studies have shown that smooth muscle myosin has three highly conserved unique loops, loop B (320-327), loop M (687-699), and loop N (125-134), similar to other myosins, skeletal muscle and dictyostelium myosins. We previously demonstrated that the effect of actin is mediated by a conformational change in one of the loops, loop M comprising amino acids 677 to 689 of skeletal muscle myosin [Maruta and Homma (1998) J. Biochem. 124, 528-533]. In the present study, in order to clarify the role of these smooth muscle myosin loops in energy transduction, we specifically labeled the loops with a fluorescent photoreactive ADP analogue, 3'-O-(N-methylanthraniloyl)-8-azido-ADP (Mant-8-N(3)-ADP), and then measured the fluorescent polarization. When Mant-8-N(3)-ADP was trapped by aluminium fluoride or vanadate into the ATPase site, Mant-8-N(3)-ADP was covalently incorporated into loop N (125-134). In contrast, Mant-8-N(3)-ADP trapped by beryllium fluoride was covalently incorporated into both loop M (687-699) and loop N (125-134) at an almost equimolar ratio. Actin binding to smooth muscle myosin S1 (SMO-S1) labeled at only loop N (125-134) increased the polarization due to the viscosity of actin. In contrast, S1 labeled at both loops N and M showed a much smaller increase in polarization. Our results indicate that the probe at loop M (687-699) of smooth muscle myosin moved to a less hindered region, suggesting that actin binding induces conformational changes at loop M (687-699) similar to those of the corresponding loop (677-689) in skeletal muscle myosin, as previously demonstrated in our laboratory.