The effects of myosin, heavy meromyosin (HMM), α-actinin, tropomyosin, and calmodulin on the interaction between an actin-depolymerizing protein (depactin) from starfish oocytes and rabbit skeletal actin were investigated.
1. Alpha-actinin or tropomyosin did not affect the inhibitory effect of depactin on salt-induced polymerization of actin and did not induce actin polymerization when added to a mixture of actin and depactin in a buffer solution which was designed to keep actin in F-form. Myosin or HMM allowed polymerization of actin under similar conditions except that ATP was not included. Addition of ATP, inorganic pyrophosphate, adenyl-5'-yl imidodiphosphate, or ADP abolished the effect of myosin.
2. None of these proteins could keep actin in a polymerized form when depactin was added to the mixture of actin and these proteins in the presence of ATP. In the absence of ATP, however, myosin protected actin against depactin.
3. The association constant between actin and depactin was estimated from competitive binding experiments using HMM to be around 4.5×106 M^-1 assuming the association constant between actin and HMM to be 3×10⁹ M^-1 (Greene, L. E. & Eisenberg, E. (1980) J. Biol. Chem. 255, 549-555).
4. Actin did not activate Mg-ATPase activity of HMM in the presence of depactin. From the above results, the mode of interaction of myosin with actin in the presence of depactin and the possible role of myosin in actin assembly in the cell are discussed.