Binding of myosin A to F-actin.

The studies of Szent-Gyorgyi (1) have clearly established the formation of actomyosin from F-actin and myosin A. SzentGyorgyi (l), Portzehl, Schramm, and Weber (2), Jaisle (3), Snellman and Gelotte (4), and Spicer and Gergely (5) have suggested from their viscometric and centrifugal studies that the maximal amount of myosin A which can be bound by F-actin is in the ratio of 2 to 4 : 1, by weight. Laki, Spicer, and Carroll (6) and Portzehl et al. (2) have reported that the binding reaction is a reversible one which depends on temperature and ionic strength. However, it seems difficult to determine, by viscometric or ultracentrifugal experiments, the binding ratio of myosin A to F-actin, since such experiments must usually be performed under conditions involving large intermolecular interactions of fibrous proteins. Gergely and Kohler (7) have recently applied the method of light scattering, which can be used with protein solutions of much lower concentration. It may, however, be pointed out that under their experimental conditions the dissociation constant for the binding reaction is so small that its accurate measurement may be very difficult. So far, no kinetic study has been made of the binding reaction of myosin A to F-actin. In the present study the velocity and equilibrium constants for the binding of myosin A to F-actin have been determined, under various experimental conditions, by use of the light scattering method. Furthermore, we have found that the optical rotatory dispersion changes slightly with the formation of actomyosin, and that actomyosin formation is inhibited when one amino group in an actin monomer is modified by the addition of trinitrobenzenesulfonate. On the basis of these results, an attempt has been made to elucidate the molecular mechanism of the reaction.