Binding of troponin-T fragments to several types of tropomyosin. Sensitivity to Ca2+ in the presence of troponin-C.

The binding of rabbit skeletal muscle troponin-T and several of its fragments to various types of tropomyosin immobilized on Sepharose 4B affinity columns equilibrated with 0.1 M NaCl, pH 7.0 buffer has been investigated. With rabbit skeletal muscle alpha-tropomyosin, intact troponin-T was eluted with an NaCl gradient at 0.42 M, while its fragments T1 (residues 1-158) and CB1 (residues 1-151) were eluted at 0.32 M NaCl in either "plus" or "minus" Ca2+ buffer in the presence of troponin-C. Fragment T2 (residues 159-259) was eluted at 0.22 M NaCl in minus Ca2+ buffer in the presence of troponin-C, but in the void volume with troponin-C under plus Ca2+ conditions. With immobilized nonpolymerizable alpha-tropomyosin, T1 was not bound, whereas T2 was eluted at the same NaCl concentration (0.21 M) as with alpha-tropomyosin. This binding was sensitive to Ca2+ in the presence of troponin-C. The results are consistent with a structural interpretation of a two-site model of troponin attachment to alpha-tropomyosin (Mak, A. S., and Smillie, L. B. (1981) J. Mol. Biol. 149, 541-550). With beta-tropomyosin from rabbit skeletal muscle and with tropomyosins from equine platelets and chicken gizzard, the binding of fragment T1 was not observed at 0.1 M NaCl, while that for T2 was the same as for rabbit skeletal alpha-tropomyosin and remained Ca2+-sensitive in the presence of troponin-C. In the case of bovine aorta tropomyosin, neither T1 nor T2 was bound under these conditions.