THIOPHOSPHORYLATION OF MYOSIN LIGHT-CHAIN INCREASES RIGOR STIFFNESS OF RABBIT SMOOTH-MUSCLE

1. The effect of thiophosphorylation of the regulatory myosin light ch ain (MLC20) on rigor stiffness was determined in permeabilized rabbit bladder smooth muscle. 2. Rigor stiffness of alpha-toxin-permeabilized smooth muscle was significantly increased by thiophosphorylation of M LC20. This increase may have been due to partial shortening (melting) in the proximal rod region and/or stiffening of the regulatory domain of the myosin head. 3. We suggest that phosphorylation of MLC20, by in creasing the stiffness of the S1 lever arm and/or S2 hinge regions of the myosin molecule, favours separation of the two phosphorylated head s and consequent deinhibition of motor domain activity.