KINETICS OF THE INTERACTION OF MYOSIN SUBFRAGMENT-L WITH G-ACTIN

The kinetics of interaction of monomeric pyrenyl-labeled G-actin with myosin subfragment-1 (S-1 (A(1)) and S-1(A(2)) isomers) has been exami ned in the stopped-flow at low ionic strength, The data confirm the pr eviously reported existence of binary GS and ternary G(2)S complexes, The increase in pyrenyl-actin fluorescence which monitors the G-actin- S1 interactions is linked to the isomerization of these complexes foll owing rapid equilibrium binding steps, The rates of isomerization are similar to 200 s(-1) for GS and similar to 50 s(-1) for G(2)S at 4 deg rees C and in the absence of ATP, DNaseI and S-1 bind G-actin essentia lly in a mutually exclusive fashion, Both GS and G(2)S are dissociated by MgATP and MgADP, The kinetics and mechanism of ATP-induced dissoci ation of G(2)S are quantitatively close to the ATP-induced dissociatio n of F-actin-S-1, which indicates the G(2)S is a good model for the F- actin-S-1 interface, GS and G(2)S display different kinetic behaviors in response to nucleotides, GS being less efficiently dissociated than G(2)S by MgATP, This result suggests that different mechanical proper ties of the crossbridge might correlate with different orientations of the myosin head and different actin/myosin binding ratios.