Isolation and characterization of covalently cross-linked actin dimer

Covalently cross-linked actin dimer was isolated from rabbit skeletal muscle F-actin reacted with phenylenebismaleimide (Knight, P., and Offer, G. (1978) Biochem. J. 175, 1023-1032). The UV spectrum of the purified cross-linked actin dimer, in a nonpolymerizing buffer, was very similar to that of native F-actin and not to the spectrum of G-actin. Cross-linked actin dimer polymerized to filaments that were indistinguishable in the electron microscope from F-actin made from native G-actin and that were similar to native F-actin in their ability to activate the Mg2+-ATPase of myosin subfragment-1. The critical concentrations of polymerization of cross-linked actin dimer in 0.5 mM and 2.0 mM MgCl2, 2 to 4 microM, and 1 to 2 microM, respectively, were similar to the values for native G-actin. Cross-linked actin dimer contained 2 mol of bound nucleotide/mol of dimer. One bound nucleotide exchanged with ATP in solution with a t 1/2 of 55 min and with ADP with a t 1/2 of 5 h. The second bound nucleotide exchanged much more slowly. The more rapidly exchangeable site contained 10 to 15% bound ADP.Pi and 85 to 90% bound ATP while the second site contained much less, if any, bound ADP.Pi. Cross-linked actin dimer had an ATPase activity in 0.5 mM MgCl2 that was 7 times greater than the ATPase activity of native G-actin and that was also stimulated by cytochalasin D. These data are discussed in relation to the possible role of ATP in actin polymerization and function with the speculation that the cross-linked actin dimer may serve simultaneously as a useful model for each of the two different ends of native F-actin.