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 m inimal (<1%) thiophosphorylation of the myosin light chain increases t he subsequent calcium sensitivity of force output and myosin light cha in phosphorylation, The change in calcium sensitivity results at least in part from a 5-fold decrease in myosin light chain phosphatase acti vity, One of the few proteins thiophosphorylated under these condition s is the 130-kDa subunit of myosin light chain phosphatase. These resu lts suggest that thiophosphorylation of this subunit leads to a decrea se in the activity of the phosphatase, and that phosphorylation and de phosphorylation of the subunit may play a role in regulating myosin li ght chain phosphatase activity.