Pressure-relaxation studies of pyrene-labelled actin and myosin subfragment 1 from rabbit skeletal muscle. Evidence for two states of acto-subfragment 1

We have used actin labelled at Cys-374 with N-(1-pyrenyl)iodoacetamide [Kouyama & Mihashi (1981) Eur. J. Biochem. 114, 33-38] to monitor pressure-induced relaxations of acto-myosin subfragment 1. This label greatly increases the sensitivity of measurement of dissociated actin and reveals the presence of two relaxations. The experimental data can be fitted by a model in which actin binds subfragment 1 relatively weakly (K = 5.9 × 10(4) M-1) and then isomerizes to a more tightly bound complex (K = 1.7 × 10(7) M-1). This directly observed isomerization supports the model of Geeves, Goody & Gutfreund [(1984) J. Muscle Res. Cell. Motil. 5, 351-361]. The rate of the isomerization is too high to be observed in the pressure-jump apparatus (less than 200 microseconds), but analysis of the amplitudes allows estimation of the equilibrium constant of the isomerization as 280 (20 degrees C, 0.1 M-KCl, pH 7). The equilibrium is sensitive to temperature, pressure, ionic strength and the presence of ethylene glycol. The pressure-sensitivity of the isomerization suggests a significant conformational change of the acto-myosin subfragment 1 complex.