Troponin (Tn) was discovered by Setsuro Ebashi in 1963. It is a globular molecule, which consists of three subunits, troponin I (TnI, which inhibits actomyosin ATPase activity), troponin C (TnC, which binds calcium), and troponin T (TnT, with is the tropomyosin binding component of Tn). TnC is the Ca2+ receptor of the thin filament and is the only Tn subunit crystallized to date. The troponin-tropomyosin complex regulates muscle contraction in response to the levels of Ca2+ ions. Only the Tn subunit TnC binds Ca2+. An allosteric mechanism is believed to regulate the binding of myosin to actin, and thus muscle contraction. In the relaxed state, tropomyosin binds along the groove in the actin double helix, and blocks the S1-binding sites of the seven actin monomers. Binding of Ca2+ to TnC causes a conformational change in TnC that is transmitted via Tn to Tm resulting in Tm moving approximately 10?deeper into the groove, exposing the myosin-binding sites. Refer to Zot and Potter (1987).
TnI inhibits the Mg2+-activated ATPase of actomyosin. TnI is a basic protein that readily complexes with the acidic TnC. In the presence of Ca2+ the affinity of TnI for TnC increases significantly. Upon muscle stimulation TnC binds Ca2+ and then complexes with TnI relieving the inhibition of the actomyosin Mg2+-ATPase by TnI.
TnC is a member of the EF hand super family of proteins and has been highly conserved through out evolution. The typical EF hand consists of a 12-membered loop, which is flanked on both sides by alpha helices containing 12 to 14 amino acid residues. The two globular domains each contain two EF hand motifs and can bind two Ca2+ molecules. The N terminal domain of TnC, is the regulatory domain of the protein. It contains 5 alpha helices. In addition to helices A and B flanking the first binding site and helices C and D flanking the second binding site, the N terminal domain contains an additional alpha helix located at the very N terminus of the protein. This N helix plays an important role in the proper alignment of the whole N-terminal domain of TnC. Without this N helix the protein does not function optimally. The C- terminal sites of TnC do not directly participate in the regulation of muscle contraction. These sites seem to play an important structural role in providing for fixation of TnC to other components of the thin filament and maintenance of the whole structure of TnC (Szczesna et al., 1996).
TnT is an asymmetric molecule that interacts with tropomyosin (Tm). Various studies suggest the binding of TnT to the C terminal end of Tm. The Tm - TnT interaction serves to fix the position of the entire Tn complex within the thin filament, so that subtle changes in the conformation of these proteins may regulate contraction. TnT also binds TnC, allowing any Ca2+-induced conformational changes in TnC to be transmitted through TnT to Tm. Recently, a highly conserved protein domain was described in the amino acid sequence of TnT (Stefancsik et al., 1998), that is characterized by a heptad repeat motif with a potential for a-helical coiled coil formation. A similar, potentially coiled coil forming domain is also conserved in all known TnI sequences, suggesting that these protein domains play a role in TnT -TnI interaction.
Ebashi, S. (1963). Third component participating in the superprecipitation of actomyosin. Nature, 200, 1010.
Vassylyev, D.G., Takeda, S., Vakatsuki, S. Maeda, K., and Maeda, Y. (1998). Crystal structure of troponin C in complex with troponin I fragments at 2.3-?resolution. Proc. Natl. Acad. Sci. USA, 95, 4847-4852.
Szczesna, D., Guzman, G., Miller, T., Zhao, Farokhi K., Ellemberger, H., and Potter, J.D. (1996) J. Biol. Chem., 271, 8381-8386.