Molecular arrangement of troponin-tropomyosin in the thin filament

Clarification of molecular arrangement of three troponin components in the thin filament is one of the essential problems in the field of the calcium regulation of muscle contraction. This review aims at discussing the present status of our structural studies of these Ca receptive proteins. Particular attention was rapid to the role of troponin T which connects troponin complex to tropomyosin. Studies on the paracrystalline structure indicated that the size of troponin complex is an order of 10 nm and the whole axial length is determined by the rod shaped troponin T molecule. The molecular arrangement of each component studied by immuno electron microscopy, indicated that troponin C, troponin I, and C-terminal region of troponin T (troponin T2) are exposed on the surface of thin filament to the similar extent. The N-terminal subfragment of troponin T, troponin T1, on the other hand, was shown to be mostly embedded in the depth of the complex and only exposed to the direction of Z-line in the thin filament. It was further attempted to connect these structural finding with the primary amino acid sequence. Since the classical crystallographic results have not yet been obtained, we instead introduced the prediction technique for the secondary and supersecondary structures from the known amino acid sequences. The interacting structure of troponin T-tropomyosin was analyzed especially in detail. It was thus concluded that the T1 region of troponin T strongly binds to tropomyosin by forming triple stranded coiled-coil structure. One plausible model of troponin components and tropomyosin constructed at amino acid level was also presented.