A specific amino acid sequence at the head-rod junction is not critical for the phosphorylation-dependent regulation of smooth muscle myosin

It has been suggested that the structure at the head-rod junction of smooth muscle myosin is important for the phosphorylation-mediated regulation of myosin motor activity. To investigate whether a specific amino acid sequence at the head-rod junction is critical for the regulation, three smooth muscle myosin mutants in which the sequence at the N-terminal end of S2 is deleted to various extents were expressed in Sf9 cells; 28, 56, and 84 amino acid residues, respectively, at the position immediately C-terminal to the invariant proline (Pro849) were deleted, and the S1 domain was directly linked to the downstream sequence of the rod. The mutant myosins were expressed, purified, and biochemically characterized. All three myosin mutants showed a stable double-headed structure based upon electron microscopic observation. Both the actin-activated ATPase activity and the actin translocating activity of the mutants were completely regulated by the phosphorylation of the regulatory light chain. The actin sliding velocity of the three mutant myosins was the same as the wild-type recombinant myosin. These results indicate that a specific amino acid sequence at the head-rod junction is not required for the regulation of smooth muscle myosin. The results also suggest that there is no functionally important interaction between the regulatory light chain and the heavy chain at the head-rod junction.