TnT, the tropomyosin (Tm) binding subunit of the troponin (Tn) complex, contains a globular COOH-terminus and a long NH2-terminal tail. The globular portion of TnT binds to troponin C (TnC; the calcium binding subunit), troponin I (TnI; the inhibitory subunit), and Tm in a calcium-dependent manner.
The NH2-terminal segment of TnT (TnT-NH2+)highly charged, and binds at the NH2-terminal/COOH-terminal overlap of two adjacent Tm molecules in a calcium-independent manner, thus providing a tether for the entire Tn complex to the thin filament during muscle activation.
The TnT-NH2+enhances the binding of the Tm to actin and thus likely facilitates the communication of movement between adjacent Tm with thin filament activation. The overlap of adjacent Tm is felt to be a critical component of thin filament-mediated cooperative activation.
