DYNAMICS OF THE MUSCLE THIN FILAMENT REGULATORY SWITCH - THE SIZE OF THE COOPERATIVE UNIT

Actin thin filaments containing bound tropomyosin (Tm) or tropomyosin. troponin (Tm.Tn) exist in two states (''off'' and ''on'') with differe nt affinities for myosin heads (S1), which results in the cooperative binding of S1. The rate of S1 binding to, and dissociating from, actin , Tm.actin, and Tm.Tn.actin, monitored by light scattering (LS), was c ompared with the rate of change in state, monitored by the excimer flu orescence (FI) of a pyrene label attached to Tm. The ATP-induced S1 di ssociation showed similar exponential decreases in LS for actin.S1, Tm .actin.S1, and Tm.Tn.actin.S1 +/- Ca2+. The Fl change, however, showed a delay that was greater for Tm.Tn.actin than Tm.actin, independent o f Ca2+. The S1 binding kinetics gave observed rate constants for the S 1-induced change in state that were 5-6 times the observed rate consta nts of S1 binding to Tm.actin, which were increased to 10-12 for Tm.Tn .actin, independent of Ca2+. The rate of the Fl signals showed that th e on/off states were in rapid equilibrium. These data indicate that th e apparent cooperative unit for Tm.actin is 5-6 actin subunits rather than the minimum structural unit size of 7, and is increased to 10-12 subunits for Tm.Tn.actin, independent of the presence of Ca2+. Thus, T m appears semi-flexible, and Tn increases communication between neighb oring structural units. A general model for the dynamic transitions in volved in muscle regulation is presented.