Smooth muscle myosin has two reactive thiols located near the C-terminal region of its motor domain, the
20
1c;converter
20
1d;, which rotates by
223c.gif"" alt=""not, vert, similar"" title=""not, vert, similar"" border=""0"">70° upon the transition from the
20
1c;nucleotide-free
20
1d; state to the
20
1c;pre-power stroke
20
1d; state. The incorporation rates of a thiol reagent,
5-(((
2-iodoacetyl)amino)ethyl)aminonaphthalene-
1-
sulfonic acid (IAEDANS), into these thiols were greatly altered by adding ATP or changing the myosin conformation. Comparisons of the myosin structures in the pre-power stroke state and the nucleotide-free state explained why the reactivity of both thiols is especially sensitive to a conformational change around the converter, and thus can be used as a sensor of the rotation of the converter. Modeling of the myosin structure in the pre-power stroke state, in which the most reactive thiol,
20
1c;SH
120
1d;, was selectively modified with IAEDANS, revealed that this label becomes an obstacle when the converter completely rotates toward its position in the pre-power stroke state, thus resulting in incomplete rotation of the converter. Therefore, we suggest that the limitation of the converter rotation by modification causes the as-yet unexplained phenomena of SH
1-modified myosin, including the inhibition of
10S myosin formation and the losses in phosphorylation-dependent regulation of the basic and actin-activated Mg
20
13;ATPase activities of myosin.