The effects of troponin T fragments T1 and T2 on the binding of nonpolymerizable tropomyosin to F-actin in the presence and absence of troponin I and troponin C.

Using a nonpolymerizable form of tropomyosin (NPTM) we have investigated the interactions between the T1 (residues 1-158) and T2 (residues 159-259) regions of troponin T and the other components of the thin filament at 50 mM KCl +/- Ca2+. Under these conditions the binding of NPTM to F-actin is fully restored by whole troponin (+/- Ca2+), and in each case, retains a residual degree of cooperativity as demonstrated by Scatchard and Hill plots. Fragment T2 alone had a small inductive effect on the interaction of NPTM with F-actin. In the presence of troponin I, this interaction is increased to a level which exceeds that observed with either component alone. The effects of T2 and troponin I are moderately (-Ca2+) and markedly (+Ca2+) reduced by troponin C. While fragment T1 alone did not promote induction, it accentuated the effects of T2 and troponin I. Since T1 does not interact with T2 or troponin I but does interact weakly with the NH2 terminus of tropomyosin and can be expected to bind weakly at the residual interaction site(s) at the COOH terminus of NPTM, the observed effects of T1 have been ascribed to the linking of neighboring NPTM molecules at their ends.