Regulated crosslinked actin filaments and the decoupling between their ATPase activity and sliding motility.

Troponin-tropomyosin complex from skeletal muscles was observed to regulate sliding movement of actin filaments on myosin molecules in a manner independent of their ATPase activity. When actin molecules were crosslinked with DSS (disuccinimidyl suberate), the myosin ATPase activity in the presence of the modified actin filaments complexed with both troponin and tropomyosin was only 10% less than that in the case of unmodified actin, and the ATPase activation was independent of calcium ions. In contrast, the sliding velocity of the modified actin filaments on myosin molecules decreased to zero below pCa 6.5. The present results indicate that troponin-tropomyosin complex regulates contractile movement of actomyosin systems through direct alternation of a mechanochemical property of the thin filaments, not through a decrease in the ATPase activity of the myosin molecules.