Interaction of rabbit skeletal muscle troponin T and F-actin at physiological ionic strength

Troponin T has been shown to interact significantly with F-actin at 150 mM KC1 by using an F-actin pelleting assay and 125I-labeled proteins. While troponin T fragment T1 (residues 1-158) fails to pellet with F-actin, fragment T2 (residues 159-259) mimics the binding properties of the intact molecule. The weak competition of T2 binding to F-actin, shown by subfragments of T2, indicates that the interaction site(s) encompass(es) an extensive segment of troponin T. The extent of pelleting of troponin T (or T2) with F-actin is only marginally altered in the binary complex troponin IT (or T2), indicating that the direct interactions either of troponin T (or T2) or of troponin I, or both, with F-actin are weakened when these components are incorporated into a binary complex. The binding of troponin T (or T2) is moderately (-Ca2+) or more extensively reduced (+Ca2+) in the presence of troponin C. The pelleting of Tn-T seen in the presence of Tn-C (-Ca2+) and Tn-I was further reduced when either Tn-I or Tn-C (-Ca2+) was added, respectively, to form a fully reconstituted Tn complex. As noted by others, whole troponin shows little sensitivity to Ca2+ in its binding to F-actin (-tropomyosin). These and other observations, taken together with the restoration of troponin IC (+/- Ca2+) binding to F-actin by troponin T, implicate a role for the interaction of troponin T and F-actin in the thin filament assembly.