Troponin and its components can restore the binding of nonpolymerizable tropomyosin to F-actin, highlighting the importance of the head-to-tail overlap region in thin filament assembly.
Nonpolymerizable tropomyosin, in which 11 residues have been quantitatively cleaved from the COOH terminus of muscle tropomyosin (TM) by enzymic digestion, does not bind to F-actin. Binding is restored in the presence of troponin (Tn) and absence of Ca2+. The binding is stronger than for intact TM alone and shows residual cooperativity. In the presence of Ca2+, the binding is at least 10-fold weaker and cooperativity is not observed. Tn-T alone is more effective than Tn-I alone in inducing nonpolymerizable TM binding. Tn-T plus Tn-I induce binding as effectively as whole Tn (without Ca2+). In the absence of Ca2+, Tn-T + Tn-C and Tn-I + Tn-C are more effective in promoting binding than in the presence of Ca2+. These observations emphasize the importance of the head to tail overlap region of TM in the cooperative interactions of the thin filament assembly. The effects of Ca2+ are largely understandable in terms of its known effects on the strength of interactions between Tn-I and TM + actin and between Tn-T and TM. The residual cooperativity observed in nonpolymerizable TM binding in the presence of Tn (without Ca2+) may indicate that the T1 fragment region (residues 1-158) of Tn-T spans the head to tail overlap gap between the neighboring nonpolymerizable TM molecules. Alternatively, or in addition, the cooperativity may arise from conformational changes transmitted through actin from one nonpolymerizable TM-Tn binding site to others.
Mak et al. (1983) studied this question.