Thiophosphorylation of the 130-kDa subunit is associated with a decreased activity of myosin light chain phosphatase in alpha-toxin-permeabilized smooth muscle.

Pretreatment of alpha-toxin-permeabilized smooth muscle with ATP gamma S (adenosine 5'-O-(thiotriphosphate)) under conditions resulting in minimal (< 1%) thiophosphorylation of the myosin light chain increases the subsequent calcium sensitivity of force output and myosin light chain phosphorylation. The change in calcium sensitivity results at least in part from a 5-fold decrease in myosin light chain phosphatase activity. One of the few proteins thiophosphorylated under these conditions is the 130-kDa subunit of myosin light chain phosphatase. These results suggest that thiophosphorylation of this subunit leads to a decrease in the activity of the phosphatase, and that phosphorylation and dephosphorylation of the subunit may play a role in regulating myosin light chain phosphatase activity.