General Vs Quasi-Repeat-Specific Interaction of Tropomyosin with TnI C-Terminus

Abstract

Although the atomic structures of actin, tropomyosin, and troponin have been solved separately, much remains unknown about their assembled structures in the thin filament. A variety of evidence indicates that the TnI C-terminus interacts with tropomyosin on the actin filament, and this interaction is a critical aspect of the regulation of muscle contraction. The present report concerns this TnI-tropomyosin interaction, and suggests that it involves the middle region of tropomyosin. Tropomyosin is a 40nm coiled coil protein, comprised of 7 successive quasi-repeating regions. To determine the involvement of these quasi repeats in TnI binding a panel of tropomysoins was studied in which different quasi repeats were deleted. C-terminal TnI peptide 131-210 (TnI C-term) had different effects on tropomyosin, depending upon the presence of different quasi repeats. When all tropomyosin quasi repeats are present, TnI C-term strengthens tropomyosin-actin binding affinity five-fold. Removal of tropomyosin quasi-repeats 2 and 3 diminishes this effect of TnI C-term slightly to 2.7 fold. In contrast, removal of tropomyosin repeats 3 and 4 completely abolished the TnI C-term effect on tropomyosin-actin binding. The behavior of tropomyosin missing repeats 2, 3 and 4 is very similar to the behavior of tropomyosin missing repeats 3 and 4. These findings suggest that in the normal thin filaments, which contain only one troponin for every tropomyosin, it is the middle, fourth quasi-repeat of tropomyosin which interacts with TnI C-terminus.