Fragments of rabbit striated muscle alpha-tropomyosin. II. Binding to troponin-T.

The interactions of a variety of large fragments of rabbit skeletal muscle alpha-tropomyosin, prepared as previously described, with troponin-T and a soluble tropomyosin-binding fragment of troponin-T (CB1) have been investigated by affinity chromatography and gel filtration. No specific interactions between NH2-terminal fragments encompassing residues 1-189 with troponin, troponin-T, or CB1 immobilized on a Sephadex 4B column could be demonstrated. Similarly, there was no interaction between these fragments and CB1 on a gel filtration column operated in 0.1 M KCl, 10 mM imidazole pH 7.0 buffer. On the other hand, all fragments encompassing residues 190-284 showed interaction with troponin-T on the affinity column and with CB1 by gel filtration. When mixtures of two fragments, one of which had the intact NH2-terminal sequence of the original tropomyosin structure and the other the intact COOH-terminal sequence, were applied to the gel filtration column, there was no indication of interaction between them. However, when CB1 was included in the mixture, a ternary complex of the three components was demonstrable. Fragments in which 10 or 12 residues at the NH2-terminal end of the alpha-tropomyosin sequence were absent showed no evidence of forming a ternary complex with CB1 and the COOH-terminal fragments. We conclude that the binding of the troponin-T fragment, CB1, to the COOH-terminal third of the alpha-tropomyosin molecule enhances head-to-tail aggregation of tropomyosin molecules either indirectly by the transmission of conformational changes to the head-to-tail overlap region or more directly by binding close to or at this region.