Characterization of the biologically important interaction between troponin C and the N-terminal region of troponin I

Comparative studies on the functional roles of N- and C-terminal regions of molluskan and vertebrate troponin-I

Vertebrate troponin regulates muscle contraction through alternative binding of the C-terminal region of the inhibitory subunit, troponin-I (TnI), to actin or troponin-C (TnC) in a Ca(2+)-dependent manner. To elucidate the molecular mechanisms of this regulation by molluskan troponin, we compared the functional properties of the recombinant fragments of Akazara scallop TnI and rabbit fast skeletal TnI. The C-terminal fragment of Akazara scallop TnI (ATnI(232-292)), which contains the inhibitory region (residues 104-115 of rabbit TnI) and the regulatory TnC-binding site (residues 116-131), bound actin-tropomyosin and inhibited actomyosin-tropomyosin Mg-ATPase. However, it did not interact with TnC, even in the presence of Ca(2+). These results indicated that the mechanism involved in the alternative binding of this region was not observed in molluskan troponin. On the other hand, ATnI(130-252), which contains the structural TnC-binding site (residues 1-30 of rabbit TnI) and the inhibitory region, bound strongly to both actin and TnC. Moreover, the ternary complex consisting of this fragment, troponin-T, and TnC activated the ATPase in a Ca(2+)-dependent manner almost as effectively as intact Akazara scallop troponin. Therefore, Akazara scallop troponin regulates the contraction through the activating mechanisms that involve the region spanning from the structural TnC-binding site to the inhibitory region of TnI. Together with the observation that corresponding rabbit TnI-fragment (RTnI(1-116)) shows similar activating effects, these findings suggest the importance of the TnI N-terminal region not only for maintaining the structural integrity of troponin complex but also for Ca(2+)-dependent activation.

Calcium-dependent protein-protein interactions induce changes in proximity relationships of Cys48 and Cys64 in chicken skeletal troponin I

The goal of this study was to relate conformational changes in the N-terminal domain of chicken troponin I (TnI) to Ca2+ activation of the actin-myosin interaction. The two cysteine residues in this region (Cys48 and Cys64) were labeled with two sulfhydryl-reactive pyrene-containing fluorophores [N-(1-pyrene)maleimide, and N-(1-pyrene)iodoacetamide]. The labeled TnI showed a typical fluorescence spectrum: two sharp peaks of monomer fluorescence and a broad peak of excimer fluorescence arising from the formation of an excited dimer (excimer). Results obtained show that forming a binary complex of labeled TnI with skeletal TnC (sTnC) in the absence of Ca2+ decreases the excimer fluorescence, indicating a separation of the two residues. This reduction in excimer fluorescence does not occur when labeled TnI is complexed with cardiac TnC (cTnC). The latter causes only partial activation of the Ca2+-dependent myofibrillar ATPase. The binding of Ca2+ to the two N-terminal sites of sTnC causes a significant decrease in excimer fluorescence and an increase in monomer fluorescence in complexes of labeled TnI with skeletal TnC or TnC/TnT, while Ca2+ binding to site II of cTnC only causes an increase in monomer fluorescence but no change in excimer fluorescence. Thus a conformational change in the N-terminal region of TnI may be necessary for full activation of muscle contraction.

Kinetic analysis of the interactions between troponin C (TnC) and troponin I (TnI) binding peptides: evidence for separate binding sites for the 'structural' N-terminus and the 'regulatory' C-terminus of TnI on TnC

The Ca(2+)/Mg(2+)-dependent interactions between TnC and TnI play a critical role in regulating the 'on' and 'off' states of muscle contraction as well as maintaining the structural integrity of the troponin complex in the off state. In the present study, we have investigated the binding interactions between the N-terminus of TnI (residues 1-40 of skeletal TnI) and skeletal TnC in the presence of Ca(2+) ions, Mg(2+) ions and in the presence of the C-terminal regulatory region peptides: TnI(96-115), TnI(96-131) and TnI(96-139). Our results show the N-terminus of TnI can bind to TnC with high affinity in the presence of Ca(2+) or Mg(2+) ions with apparent equilibrium dissociation constants of K(d(Ca(2+) ) ) = 48 nM and K(d(Mg(2+) ) ) = 29 nM. The apparent association and dissociation rate constants for the interactions were, k(on) = 4.8 x 10(5) M (-1) s(-1), 3.4 x 10(5) M (-1) s(-1) and k(off) = 2.3 x 10(-2) s(-1), 1.0 x 10(-2) s(-1) for TnC(Ca(2+)) and TnC(Mg(2+)) states, respectively. Competition studies between each of the TnI regions and TnC showed that both TnI regions can bind simultaneously to TnC while native gel electrophoresis and SEC confirmed the formation of stable ternary complexes between TnI(96-139) (or TnI(96-131)) and TnC-TnI(1-40). Further analysis of the binding interactions in the ternary complex showed the binding of the TnI regulatory region to TnC was critically dependent upon the presence of both TnC binding sites (i.e. TnI(96-115) and TnI(116-131)) and the presence of Ca(2+). Furthermore, the presence of TnI(1-40) slightly weakened the affinity of the regulatory peptides for TnC. Taken together, these results support the model for TnI-TnC interaction where the N-terminus of TnI remains bound to the C-domain of TnC in the presence of high and low Ca(2+) levels while the TnI regulatory region (residues 96-139) switches in its binding interactions between the actin-tropomyosin thin filament and its own sites on the N- and C-domain of TnC at high Ca(2+) levels, thus regulating muscle contraction.

Preparative reversed-phase liquid chromatography of proteins from rabbit skeletal troponin, a multi-protein complex

A reversed-phase high-performance liquid chromatography protocol for purification of all proteins in a multi-protein (TnI, TnC, TnT, tropomyosin) complex from rabbit skeletal muscle has been developed, enabling efficient purification of sample amounts ranging from 43 mg of protein complex on a standard analytical column, to 1400 mg on a column of 21.2 mm I.D. and finally, to 5700 mg on a column of 50 mm I.D. Due to problems associated with scale-up procedures for these proteins (e.g. aggregation and/or solubility issues), an initial sample fractionation was devised whereby 50% of the TnC component was precipitated with acetonitrile prior to sample introduction on the RPLC column. By subsequently taking advantage of sample overload conditions to enhance the displacement effect between sample components, coupled with very slow gradient conditions (0.1% acetonitrile/min), we were able to achieve excellent protein separations at high yields of purified proteins.