The interaction of maleimidobenzoyl G-actin with myosin subfragment 1 in solution: characterization of the MgATPase activity after chemical crosslinking.

As is well known, the ATPase/structural interactions between the S-1 moieties of myosin molecules ("cross bridges") and actin molecules in polymerized ("F") form are thought to underlie muscle contraction. It is surmised that such interactions are unitary (1 S-1:1 actin), but actual demonstration thereof is handicapped by intrinsic properties of the proteins. It is known that monomeric ("G" form) actin binds to S-1 and that in this contact only the 633-642 region of S-1 is involved; however, such unions do not activate S-1 ATPase. Recently, Bettache et al. (1989) [Proc. Natl. Acad. Sci. USA 86, 6028-6032] showed that chemically modified actin can be kept in monomeric form, and makes a stable unitary complex with S-1, but without activating S-1 ATPase. In this paper, however, we show that when such complexes are covalently crosslinked by 1-ethyl-3-[3-(dimethylamino)-propyl]carbodiimide they recover activated ATPase. The existence of such activated complexes proves an excellent model system as a soluble analogue of the F-actin-(S-1) complex.