Structure of the troponin complex. Implications of photocross-linking of troponin I to troponin C thiol mutants

Structural basis for Ca2+-regulated muscle relaxation at interaction sites of troponin with actin and tropomyosin

Troponin and tropomyosin on actin filaments constitute a Ca2+-sensitive switch that regulates the contraction of vertebrate striated muscle through a series of conformational changes within the actin-based thin filament. Troponin consists of three subunits: an inhibitory subunit (TnI), a Ca2+-binding subunit (TnC), and a tropomyosin-binding subunit (TnT). Ca2+-binding to TnC is believed to weaken interactions between troponin and actin, and triggers a large conformational change of the troponin complex. However, the atomic details of the actin-binding sites of troponin have not been determined. Ternary troponin complexes have been reconstituted from recombinant chicken skeletal TnI, TnC, and TnT2 (the C-terminal region of TnT), among which only TnI was uniformly labelled with 15N and/or 13C. By applying NMR spectroscopy, the solution structures of a "mobile" actin-binding domain (approximately 6.1 kDa) in the troponin ternary complex (approximately 52 kDa) were determined. The mobile domain appears to tumble independently of the core domain of troponin. Ca2+-induced changes in the chemical shift and line shape suggested that its tumbling was more restricted at high Ca2+ concentrations. The atomic details of interactions between actin and the mobile domain of troponin were defined by docking the mobile domain into the cryo-electron microscopy (cryo-EM) density map of thin filament at low [Ca2+]. This allowed the determination of the 3D position of residue 133 of TnI, which has been an important landmark to incorporate the available information. This enabled unique docking of the entire globular head region of troponin into the thin filament cryo-EM map at a low Ca2+ concentration. The resultant atomic model suggests that troponin interacted electrostatically with actin and caused the shift of tropomyosin to achieve muscle relaxation. An important feature is that the coiled-coil region of troponin pushed tropomyosin at a low Ca2+ concentration. Moreover, the relationship between myosin and the mobile domain on actin filaments suggests that the latter works as a fail-safe latch.

Ca(2+)-regulated structural changes in troponin

Troponin senses Ca2+ to regulate contraction in striated muscle. Structures of skeletal muscle troponin composed of TnC (the sensor), TnI (the regulator), and TnT (the link to the muscle thin filament) have been determined. The structure of troponin in the Ca(2+)-activated state features a nearly twofold symmetrical assembly of TnI and TnT subunits penetrated asymmetrically by the dumbbell-shaped TnC subunit. Ca ions are thought to regulate contraction by controlling the presentation to and withdrawal of the TnI inhibitory segment from the thin filament. Here, we show that the rigid central helix of the sensor binds the inhibitory segment of TnI in the Ca(2+)-activated state. Comparison of crystal structures of troponin in the Ca(2+)-activated state at 3.0 angstroms resolution and in the Ca(2+)-free state at 7.0 angstroms resolution shows that the long framework helices of TnI and TnT, presumed to be a Ca(2+)-independent structural domain of troponin are unchanged. Loss of Ca ions causes the rigid central helix of the sensor to collapse and to release the inhibitory segment of TnI. The inhibitory segment of TnI changes conformation from an extended loop in the presence of Ca2+ to a short alpha-helix in its absence. We also show that Anapoe, a detergent molecule, increases the contractile force of muscle fibers and binds specifically, together with the TnI switch helix, in a hydrophobic pocket of TnC upon activation by Ca ions.

Mapping contacts between regulatory domains of skeletal muscle TnC and TnI by analyses of single-chain chimeras

The troponin (Tn) complex is formed by TnC, TnI and TnT and is responsible for the calcium-dependent inhibition of muscle contraction. TnC and TnI interact in an antiparallel fashion in which the N domain of TnC binds in a calcium-dependent manner to the C domain of TnI, releasing the inhibitory effect of the latter on the actomyosin interaction. While the crystal structure of the core cardiac muscle troponin complex has been determined, very little high resolution information is available regarding the skeletal muscle TnI-TnC complex. With the aim of obtaining structural information regarding specific contacts between skeletal muscle TnC and TnI regulatory domains, we have constructed two recombinant chimeric proteins composed of the residues 1-91 of TnC linked to residues 98-182 or 98-147 of TnI. The polypeptides were capable of binding to the thin filament in a calcium-dependent manner and to regulate the ATPase reaction of actomyosin. Small angle X-ray scattering results showed that these chimeras fold into compact structures in which the inhibitory plus the C domain of TnI, with the exception of residues 148-182, were in close contact with the N-terminal domain of TnC. CD and fluorescence analysis were consistent with the view that the last residues of TnI (148-182) are not well folded in the complex. MS analysis of fragments produced by limited trypsinolysis showed that the whole TnC N domain was resistant to proteolysis, both in the presence and in the absence of calcium. On the other hand the TnI inhibitory and C-terminal domains were completely digested by trypsin in the absence of calcium while the addition of calcium results in the protection of only residues 114-137.

Mutations of hydrophobic residues in the N-terminal domain of troponin C affect calcium binding and exchange with the troponin C-troponin I96-148 complex and muscle force production

Interactions between troponin C and troponin I play a critical role in the regulation of skeletal muscle contraction and relaxation. We individually substituted 27 hydrophobic Phe, Ile, Leu, Val, and Met residues in the regulatory domain of the fluorescent troponin C(F29W) with polar Gln to examine the effects of these mutations on: (a) the calcium binding and dynamics of troponin C(F29W) complexed with the regulatory fragment of troponin I (troponin I(96-148)) and (b) the calcium sensitivity of force production. Troponin I(96-148) was an accurate mimic of intact troponin I for measuring the calcium dynamics of the troponin C(F29W)-troponin I complexes. The calcium affinities of the troponin C(F29W)-troponin I(96-148) complexes varied approximately 243-fold, whereas the calcium association and dissociation rates varied approximately 38- and approximately 33-fold, respectively. Interestingly, the effect of the mutations on the calcium sensitivity of force development could be better predicted from the calcium affinities of the troponin C(F29W)-troponin I(96-148) complexes than from that of the isolated troponin C(F29W) mutants. Most of the mutations did not dramatically affect the affinity of calcium-saturated troponin C(F29W) for troponin I(96-148). However, the Phe(26) to Gln and Ile(62) to Gln mutations led to >10-fold lower affinity of calcium-saturated troponin C(F29W) for troponin I(96-148), causing a drastic reduction in force recovery, even though these troponin C(F29W) mutants still bound to the thin filaments. In conclusion, elucidating the determinants of calcium binding and exchange with troponin C in the presence of troponin I provides a deeper understanding of how troponin C controls signal transduction.

Ca2+-induced conformational transition in the inhibitory and regulatory regions of cardiac troponin I

Cardiac muscle activation is initiated by the binding of Ca(2+) to the single N-domain regulatory site of cardiac muscle troponin C (cTnC). Ca(2+) binding causes structural changes between cTnC and two critical regions of cardiac muscle troponin I (cTnI): the regulatory region (cTnI-R, residues 150-165) and the inhibitory region (cTnI-I, residues130-149). These changes are associated with a decreased cTnI affinity for actin and a heightened affinity for cTnC. Using Förster resonance energy transfer, we have measured three intra-cTnI distances in the deactivated (Mg(2+)-saturated) and Ca(2+)-activated (Ca(2+)-saturated) states in reconstituted binary (cTnC-cTnI) and ternary (cTnC-cTnI-cTnT) troponin complexes. Distance A (spanning cTnI-R) was unaltered by Ca(2+). Distances B (spanning both cTnI-R and cTnI-I) and C (from a residue flanking cTnI-I to a residue in the center of cTnI-R) exhibited Ca(2+)-induced increases of >8 A. These results compliment our previous determination of the distance between residues flanking cTnI-I alone. Together, the data suggest that Ca(2+) activation causes residues within cTnI-I to switch from a beta-turn/coil to an extended quasi-alpha-helical conformation as the actin-contacts are broken, whereas cTnI-R remains alpha-helical in both Mg(2+)- and Ca(2+)-saturated states. We have used the data to construct a structural model of the cTnI inhibitory and regulatory regions in the Mg(2+)- and Ca(2+)-saturated states.

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.

Engineering competitive magnesium binding into the first EF-hand of skeletal troponin C

The goal of this study was to examine the mechanism of magnesium binding to the regulatory domain of skeletal troponin C (TnC). The fluorescence of Trp(29), immediately preceding the first calcium-binding loop in TnC(F29W), was unchanged by addition of magnesium, but increased upon calcium binding with an affinity of 3.3 microm. However, the calcium-dependent increase in TnC(F29W) fluorescence could be reversed by addition of magnesium, with a calculated competitive magnesium affinity of 2.2 mm. When a Z acid pair was introduced into the first EF-hand of TnC(F29W), the fluorescence of G34DTnC(F29W) increased upon addition of magnesium or calcium with affinities of 295 and 1.9 microm, respectively. Addition of 3 mm magnesium decreased the calcium sensitivity of TnC(F29W) and G34DTnC(F29W) approximately 2- and 6-fold, respectively. Exchange of G34DTnC(F29W) into skinned psoas muscle fibers decreased fiber calcium sensitivity approximately 1.7-fold compared with TnC(F29W) at 1 mm [magnesium](free) and approximately 3.2-fold at 3 mm [magnesium](free). Thus, incorporation of a Z acid pair into the first EF-hand allows it to bind magnesium with high affinity. Furthermore, the data suggests that the second EF-hand, but not the first, of TnC is responsible for the competitive magnesium binding to the regulatory domain.

Kinetic analysis of the interactions between troponin C and the C-terminal troponin I regulatory region and validation of a new peptide delivery/capture system used for surface plasmon resonance

Using surface plasmon resonance (SPR)-based biosensor analysis and fluorescence spectroscopy, the apparent kinetic constants, k(on) and k(off), and equilibrium dissociation constant, K(d), have been determined for the binding interaction between rabbit skeletal troponin C (TnC) and rabbit skeletal troponin I (TnI) regulatory region peptides: TnI(96-115), TnI(96-131) and TnI(96-139). To carry out SPR analysis, a new peptide delivery/capture system was utilized in which the TnI peptides were conjugated to the E-coil strand of a de novo designed heterodimeric coiled-coil domain. The TnI peptide conjugates were then captured via dimerization to the opposite strand (K-coil), which was immobilized on the biosensor surface. TnC was then injected over the biosensor surface for quantitative binding analysis. For fluorescence spectroscopy analysis, the environmentally sensitive fluoroprobe 5-((((2-iodoacetyl)amino)ethyl)amino) naphthalene-1-sulfonic acid (1,5-IAEDANS) was covalently linked to Cys98 of TnC and free TnI peptides were added. SPR analysis yielded equilibrium dissociation constants for TnC (plus Ca(2+)) binding to the C-terminal TnI regulatory peptides TnI(96-131) and TnI(96-139) of 89nM and 58nM, respectively. The apparent association and dissociation rate constants for each interaction were k(on)=2.3x10(5)M(-1)s(-1), 2.0x10(5)M(-1)s(-1) and k(off)=2.0x10(-2)s(-1), 1.2x10(-2)s(-1) for TnI(96-131) and TnI(96-139) peptides, respectively. These results were consistent with those obtained by fluorescence spectroscopy analysis: K(d) being equal to 130nM and 56nM for TnC-TnI(96-131) and TnC-TnI(96-139), respectively. Interestingly, although the inhibitory region peptide (TnI(96-115)) was observed to bind with an affinity similar to that of TnI(96-131) by fluorescence analysis (K(d)=380nM), its binding was not detected by SPR. Subsequent investigations examining salt effects suggested that the binding mechanism for the inhibitory region peptide is best characterized by an electrostatically driven fast on-rate ( approximately 1x10(8) to 1x10(9)M(-1)s(-1)) and a fast off-rate ( approximately 1x10(2)s(-1)). Taken together, the determination of these kinetic rate constants permits a clearer view of the interactions between the TnC and TnI proteins of the troponin complex.

Structure of the inhibitory region of troponin by site directed spin labeling electron paramagnetic resonance

Site-directed spin labeling EPR (SDSL-EPR) was used to determine the structure of the inhibitory region of TnI in the intact cardiac troponin ternary complex. Maeda and collaborators have modeled the inhibitory region of TnI (skeletal 96-112: the structural motif that communicates the Ca(2+) signal to actin) as a kinked alpha-helix [Vassylyev, D., Takeda, S., Wakatsuki, S., Maeda, K. & Maeda, Y. (1998) Proc. Natl. Acad. Sci. USA 95, 4847-4852), whereas Trewhella and collaborators have proposed the same region to be a flexible beta-hairpin [Tung, C. S., Wall, M. E., Gallagher, S. C. & Trewhella, J. (2000) Protein Sci. 9, 1312-1326]. To distinguish between the two models, residues 129-145 of cardiac TnI were mutated sequentially to cysteines and labeled with the extrinsic spin probe, MTSSL. Sequence-dependent solvent accessibility was measured as a change in power saturation of the spin probe in the presence of the relaxation agent. In the ternary complex, the 129-137 region followed a pattern characteristic of a regular 3.6 residues/turn alpha-helix. The following region, residues 138-145, showed no regular pattern in solvent accessibility. Measurements of 4 intradomain distances within the inhibitory sequence, using dipolar EPR, were consistent with an alpha-helical structure. The difference in side-chain mobility between the ternary (C.I.T) and binary (C.I) complexes revealed a region of interaction of TnT located at the N-terminal end of the inhibitory sequence, residues 130-135. The above findings for the troponin complex in solution do not support either of the computational models of the binary complex; however, they are in very good agreement with a preliminary report of the x-ray structure of the cardiac ternary complex [Takeda, S. Yamashita, A., Maeda, K. & Maeda, Y. (2002) Biophys. J. 82, 832].

Proximity relationships between residue 117 of rabbit skeletal troponin-I and residues in troponin-C and actin

We used resonance energy transfer and site-directed photo-cross-linking to probe the Ca(2+)-dependent proximity relationships between residue 117 next to the C-terminus of the inhibitory region in rabbit skeletal troponin-I (TnI) and residues in troponin-C (TnC) and in actin. A mutant TnI that contains a single cysteine at position 117 (I117) was constructed, and the distance between TnI residue 117 and TnC residue 98 was measured with the following results: for both the binary TnC-TnI complex and the ternary troponin complex, this distance was 30 and 41 A in the presence and absence of Ca(2+), respectively. The distance between TnI residue 117 and Cys374 of actin was 48 and 41 A in the presence and absence of Ca(2+), respectively. Six additional distances from this TnI residue to cysteines in TnC mutants were measured and used to localize this residue with respect to the crystal structure of TnC. The results show that in the presence of Ca(2+) it is localized near the B and C helices of TnC's N-terminal domain. In the absence of Ca(2+) this residue moves away from this location by approximately 8 A. Photo-cross-linking experiments show that I117 labeled with 4-maleimidobenzophenone photo-cross-linked to TnC but not to actin in both the presence and absence of Ca(2+). Taken together these results provide independent experimental support for the proposal (Y. Luo, J. L. Wu, B. Li, K. Langsetmo, J. Gergely, and T. Tao, 2000, J. Mol. Biol. 296:899-910) that upon Ca(2+) removal the region comprising TnI residues 114-125 triggers the movements of residues 89-113 and 130-150 toward actin, but does not itself interact with actin.

Calcium- and magnesium-dependent interactions between the C-terminus of troponin I and the N-terminal, regulatory domain of troponin C

The muscle thin filament protein troponin (Tn) regulates contraction of vertebrate striated muscle by conferring Ca2+ sensitivity to the interaction of actin and myosin. Troponin C (TnC), the Ca2+ binding subunit of Tn contains two homologous domains and four divalent cation binding sites. Two structural sites in the C-terminal domain of TnC bind either Ca2+ or Mg2+, and two regulatory sites in the N-terminal domain are specific for Ca2+. Interactions between TnC and the inhibitory Tn subunit troponin I (TnI) are of central importance to the Ca2+ regulation of muscle contraction and have been intensively studied. Much remains to be learned, however, due mainly to the lack of a three-dimensional structure for TnI. In particular, the role of amino acid residues near the C-terminus of TnI is not well understood. In this report, we prepared a mutant TnC which contains a single Trp-26 residue in the N-terminal, regulatory domain. We used fluorescence lifetime and quenching measurements to monitor Ca2+- and Mg2+-dependent changes in the environment of Trp-26 in isolated TnC, as well as in binary complexes of TnC with a Trp-free mutant of TnI or a truncated form of this mutant, TnI(1-159), which lacked the C-terminal 22 amino acid residues of TnI. We found that full-length TnI and TnI(1-159) affected Trp-26 similarly when all four binding sites of TnC were occupied by Ca2+. When the regulatory Ca2+-binding sites in the N-terminal domain of TnC were vacant and the structural sites in the C-terminal domain of were occupied by Mg2+, we found significant differences between full-length TnI and TnI(1-159) in their effect on Trp-26. Our results provide the first indica- tion that the C-terminus of TnI may play an important role in the regulation of vertebrate striated muscle through Ca2+-dependent interactions with the regula- tory domain of TnC.

Conformations of the regulatory domain of cardiac troponin C examined by residual dipolar couplings

Conformations of the regulatory domain of cardiac troponin C (cNTnC) were studied by means of residual dipolar couplings measured from samples dissolved in dilute liquid crystals. Changes in the main chain HN residual dipolar couplings revealed a conformational change in cNTnC due to the complexation with the second binding region (amino acids 148-163) of cardiac troponin I (cTnI). Formation of the complex is accompanied with a molecular realignment in the liquid crystal. The residual dipolar couplings measured for apo-cNTnC and the complex with TnI were in agreement with the values computed from the corresponding closed and open solution structures, whereas for the calcium-loaded conformation the correlation and quality factor were only modest. Ca2+-cNTnC may be subject to conformational exchange. The data support the model that cardiac troponin C functions as a calcium-dependent open-closed switch, such as the skeletal troponin C.

A model of troponin-I in complex with troponin-C using hybrid experimental data: the inhibitory region is a beta-hairpin

We present a model for the skeletal muscle troponin-C (TnC)/troponin-I (TnI) interaction, a critical molecular switch that is responsible for calcium-dependent regulation of the contractile mechanism. Despite concerted efforts by multiple groups for more than a decade, attempts to crystallize troponin-C in complex with troponin-I, or in the ternary troponin-complex, have not yet delivered a high-resolution structure. Many groups have pursued different experimental strategies, such as X-ray crystallography, NMR, small-angle scattering, chemical cross-linking, and fluorescent resonance energy transfer (FRET) to gain insights into the nature of the TnC/TnI interaction. We have integrated the results of these experiments to develop a model of the TnC/TnI interaction, using an atomic model of TnC as a scaffold. The TnI sequence was fit to each of two alternate neutron scattering envelopes: one that winds about TnC in a left-handed sense (Model L), and another that winds about TnC in a right-handed sense (Model R). Information from crystallography and NMR experiments was used to define segments of the models. Tests show that both models are consistent with available cross-linking and FRET data. The inhibitory region TnI(95-114) is modeled as a flexible beta-hairpin, and in both models it is localized to the same region on the central helix of TnC. The sequence of the inhibitory region is similar to that of a beta-hairpin region of the actin-binding protein profilin. This similarity supports our model and suggests the possibility of using an available profilin/actin crystal structure to model the TnI/actin interaction. We propose that the beta-hairpin is an important structural motif that communicates the Ca2+-activated troponin regulatory signal to actin.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Calcium-induced flexibility changes in the troponin C-troponin I complex

The contraction of vertebrate striated muscle is modulated by Ca(2+) binding to the regulatory protein troponin C (TnC). Ca(2+) binding causes conformational changes in TnC which alter its interaction with the inhibitory protein troponin I (TnI), initiating the regulatory process. We have used the frequency domain method of fluorescence resonance energy transfer (FRET) to measure distances and distance distributions between specific sites in the TnC-TnI complex in the presence and absence of Ca(2+) or Mg(2+). Using sequences based on rabbit skeletal muscle proteins, we prepared functional, binary complexes of wild-type TnC and a TnI mutant which contains no Cys residues and a single Trp residue at position 106 within the TnI inhibitory region. We used TnI Trp-106 as the FRET donor, and we introduced energy acceptor groups into TnC by labeling at Met-25 with dansyl aziridine or at Cys-98 with N-(iodoacetyl)-N'-(1-sulfo-5-naphthyl)ethylenediamine. Our distance distribution measurements indicate that the TnC-TnI complex is relatively rigid in the absence of Ca(2+), but becomes much more flexible when Ca(2+) binds to regulatory sites in TnC. This increased flexibility may be propagated to the whole thin filament, helping to release the inhibition of actomyosin ATPase activity and allowing the muscle to contract. This is the first report of distance distributions between TnC and TnI in their binary complex.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Cardiac troponin I inhibitory peptide: location of interaction sites on troponin C

Cardiac troponin I(129-149) binds to the calcium saturated cardiac troponin C/troponin I(1-80) complex at two distinct sites. Binding of the first equivalent of troponin I(129-149) was found to primarily affect amide proton chemical shifts in the regulatory domain, while the second equivalent perturbed amide proton chemical shifts within the D/E linker region. Nitrogen-15 transverse relaxation rates showed that binding the first equivalent of inhibitory peptide to the regulatory domain decreased conformational exchange in defunct calcium binding site I and that addition of the second equivalent of inhibitory peptide decreased flexibility in the D/E linker region. No interactions between the inhibitory peptide and the C-domain of cardiac troponin C were detected by these methods demonstrating that the inhibitory peptide cannot displace cTnI(1-80) from the C-domain.

Photocrosslinking of benzophenone-labeled single cysteine troponin I mutants to other thin filament proteins

The interaction sites of rabbit skeletal troponin I (TnI) with troponin C (TnC), troponin T (TnT), tropomyosin (Tm) and actin were mapped systematically using nine single cysteine residue TnI mutants with mutation sites at positions 6, 48, 64, 89, 104, 121, 133, 155 or 179 (TnI6, TnI48 etc.). Each mutant was labeled with the heterobifunctional photocrosslinker 4-maleimidobenzophenone (BP-Mal), and incorporated into the TnI.TnC binary complex, the TnI.TnC.TnT ternary troponin (Tn) complex, and the Tn.Tm.F-actin synthetic thin filament. Photocrosslinking reactions carried out in the presence and absence of Ca(2+) yielded the following results: (1) BP-TnI6 photocrosslinked primarily to TnC with a small degree of Ca(2+)-dependence in all the complex forms. (2) BP-TnI48, TnI64 and TnI89 photocrosslinked to TnT with no Ca(2+)-dependence. Photocrosslinking to TnC was reduced in the ternary versus the binary complex. BP-TnI89 also photocrosslinked to actin with higher yields in the absence of Ca(2+) than in its presence. (3) BP-TnI104 and TnI133 photocrosslinked to actin with much higher yields in the absence than in the presence of Ca(2+). (4) BP-TnI121 photocrosslinked to TnC with a small degree of Ca(2+)-dependence, and did not photocrosslink to actin. (5) BP-TnI155 and TnI179 photocrosslinked to TnC, TnT and actin, but all with low yields. All the labeled mutants photocrosslinked to TnC with varying degrees of Ca(2+)-dependence, and none to Tm. These results, along with those published allowed us to construct a structural and functional model of TnI in the Tn complex: in the presence of Ca(2+), residues 1-33 of TnI interact with the C-terminal domain hydrophobic cleft of TnC, approximately 48-89 with TnT, approximately 90-113 with TnC's central helix, approximately 114-125 with TnC's N-terminal domain hydrophobic cleft, and approximately 130-150 with TnC's A-helix. In the absence of Ca(2+), residues approximately 114-125 move out of TnC's N-terminal domain hydrophobic cleft and trigger the movements of residues approximately 89-113 and approximately 130-150 away from TnC and towards actin.

Conformational changes induced in troponin I by interaction with troponin T and actin/tropomyosin

Troponin I (TnI) is the inhibitory component of the striated muscle Ca2+ regulatory protein troponin (Tn). The other two components of Tn are troponin C (TnC), the Ca2+-binding component, and troponin T (TnT), the tropomyosin-binding component. We have used limited chymotryptic digestion to probe the local conformation of TnI in the free state, the binary TnC*TnI complex, the ternary TnC*. TnI*TnT (Tn) complex, and in the reconstituted Tn*tropomyosin*F-actin filament. The digestion of TnI alone or in the TnC*TnI complex produced initially two major fragments via a cleavage of the peptide bond between Phe100 and Asp101 in the so-called inhibitory region. In the ternary Tn complex cleavage occurred at a new site between Leu140 and Lys141. In the absence of Ca2+ this was followed by digestion of the 1-140 fragment at Leu122 and Met116. In the reconstituted thin filament the same fragments as in the case of the ternary complex were produced, but the rate of digestion was slower in the absence than in the presence of Ca2+. These results indicate firstly that in both free TnI and TnI complexed with TnC there is an exposed and flexible site in the inhibitory region. Secondly, TnT affects the conformation of TnI in the inhibitory region and also in the region that contains the 140-141 bond. Thirdly, the 140-141 region of TnI is likely to interact with actin in the reconstituted thin filament when Ca2+ is absent. These findings are discussed in terms of the role of TnI in the mechanism of thin filament regulation, and in light of our previous results [Y. Luo, J.-L. Wu, J. Gergely, T. Tao, Biochemistry 36 (1997) 13449-13454] on the global conformation of TnI.

Mapping subdomains in the C-terminal region of troponin I involved in its binding to troponin C and to thin filament

Troponin I (TnI) is the inhibitory component of troponin, the ternary complex that regulates skeletal and cardiac muscle contraction. Previous work showed that the C-terminal region of TnI, when linked to the "inhibitory region" (residues 98-116), possesses the major regulatory functions of the molecule (Farah, C. S., Miyamoto, C. A., Ramos, C. H. I., Silva, A. C. R., Quaggio, R. B., Fujimori, K., Smillie, L. B., and Reinach, F. C. (1994) J. Biol. Chem. 269, 5230-5240). To investigate these functions in more detail, serial deletion mutants of the C-terminal region of TnI were constructed. These experiments showed that longer C-terminal deletions result in lower inhibition of the actomyosin ATPase activity and weaken the interaction with the N-terminal domain of troponin C (TnC), consistent with the antiparallel model for the interaction between these two proteins. The conclusion is that the whole C-terminal region of TnI is necessary for its full regulatory activity. The region between residues 137 and 144, which was shown to have homology with residues 108-115 in the inhibitory region (Farah, C. S., and Reinach, F. C. (1995) FASEB J. 9, 755-767), is involved in the binding to TnC. The region between residues 98 and 129 is involved in modulating the affinity of TnC for calcium. The C-terminal residues 166-182 are involved in the binding of TnI to thin filament. A model for the function of TnI is discussed.

Ca2+-dependent interaction of the inhibitory region of troponin I with acidic residues in the N-terminal domain of troponin C

Ca2+ regulation of vertebrate striated muscle contraction is initiated by conformational changes in the N-terminal, regulatory domain of the Ca2+-binding protein troponin C (TnC), altering the interaction of TnC with the other subunits of troponin complex, TnI and TnT. We have investigated the role of acidic amino acid residues in the N-terminal, regulatory domain of TnC in binding to the inhibitory region (residues 96-116) of TnI. We constructed three double mutants of TnC (E53A/E54A, E60A/E61A and E85A/D86A), in which pairs of acidic amino acid residues were replaced by neutral alanines, and measured their affinities for synthetic inhibitory peptides. These peptides had the same amino acid sequence as TnI segments 95-116, 95-119 or 95-124, except that the natural Phe-100 of TnI was replaced by a tryptophan residue. Significant Ca2+-dependent increases in the affinities of the two longer peptides, but not the shortest one, to TnC could be detected by changes in Trp fluorescence. In the presence of Ca2+, all the mutant TnCs showed about the same affinity as wild-type TnC for the inhibitory peptides. In the presence of Mg2+ and EGTA, the N-terminal, regulatory Ca2+-binding sites of TnC are unoccupied. Under these conditions, the affinity of TnC(E85A/D86A) for inhibitory peptides was about half that of wild-type TnC, while the other two mutants had about the same affinity. These results imply a Ca2+-dependent change in the interaction of TnC Glu-85 and/or Asp-86 with residues (117-124) on the C-terminal side of the inhibitory region of TnI. Since Glu-85 and/or Asp-86 of TnC have also been demonstrated to be involved in Ca2+-dependent regulation through interaction with TnT, this region of TnC must be critical for troponin function.

Molecular switches in troponin

The contraction of vertebrate striated muscle contraction, and hence its work output, is controlled by Ca2+, which binds to troponin (Tn) associated with tropomyosin (TM) and actin in the thin filaments. Tn consists of three subunits: TnC, the Ca(2+)-receptor; TnI, an inhibitor of actomyosin activity; and TnT, anchoring Tn to TM. Of the four Ca(2+)-binding sites, I and II in the N-terminal domain are Ca-specific sites, while sites III and IV, the high affinity Ca-Mg sites, are in the C-domain. The former are recognized as the functionally important triggering sites. TnC, whose structure has been solved by X-ray crystallography and recently by high-resolution NMR, contains two homologous globular domains connected by an unusual single alpha-helix. The C-terminal domain exhibits an open hydrophobic area regardless of whether Ca2+ or Mg2+ is bound to sites III and IV. In contrast, the N-terminal domain is a closed structure that opens a hydrophobic patch upon Ca(2+)-binding to its two "triggering" sites producing a TnI binding area. Crosslinking and fragment binding studies indicate that, in the main, the two polypeptide chains run in opposite directions in the complex of TnC with Tn. A model of TnC-TnI interactions based on low angle X-ray and neutron scattering is discussed in light of biochemical and other physico-chemical studies. The opening of the structure in the N-terminal domain of TnC may be regarded as a molecular switch. It activates a molecular switch in TnI, reflected in the movement of portions of its C-terminal half, including Cys 133, away from actin and closer to TnC, as well as other structural changes in TnI. Finally the role of TnT in switching and transmitting the Ca(2+)-signal is discussed.

Mutations in the N- and D-helices of the N-domain of troponin C affect the C-domain and regulatory function

Troponin C contains a 14-residue alpha-helix at the amino terminus, the N-helix, that calmodulin lacks. Deletion of the first 11-14 residues of troponin C alters function. In the present investigation a mutant lacking residues 1-7 of the N-helix has normal conformation, Ca2+ binding, and regulatory function. Thus, residues 8-14 of the N-helix are generally sufficient for troponin C function. In the x-ray structures of troponin C there is a salt bridge between Arg 11 in the N-helix and Glu 76 in the D-helix. Destroying the salt bridge by individually mutating the residues to Cys has no effect on function. However, mutation of both residues to Cys reduces troponin C's affinity for the troponin complex on the thin filament, reduces the stability of the N-domain in the absence of divalent cations, increases the Ca2+ affinity and reduces the cooperativity of the Ca2+Mg2+ sites in the C-domain, and alters the conformational change that takes place upon Ca2+ binding (but not Mg2+ binding) to the C-domain. Cross-linking with bis-(maleimidomethylether) partially restores function. The Ca2+-specific sites in the N-domain, those closest to the sites of the mutations, are unaffected in the assays employed. These results show that the N-helix is a critical structural element for interaction with and activation of the thin filament. Moreover, mutations in the N-helix affect the C-terminal domain, consistent with recent structural studies showing that the N-helix and C-terminal domain are physically close.

Heterogeneity of Atlantic salmon troponin-I

Three non-identical, full length troponin-I (Tn-I) clones were isolated from an Atlantic salmon myotomal (trunk) muscle cDNA library. The primary structures, which are predicted to range from 172 to 180 amino acids in length, exhibit similar percent identity scores when compared with fast, slow and cardiac specific Tn-Is from higher vertebrates. When the sequence data are considered along with the results of Western blotting it is evident that Tn-I is more heterogeneous in Atlantic salmon than has been previously shown in higher vertebrates.

Localization of Cys133 of rabbit skeletal troponin-I with respect to troponin-C by resonance energy transfer

We have used the technique of resonance energy transfer in conjunction with distance geometry analysis to localize Cys133 of troponin-I (TnI) with respect to troponin-C (TnC) in the ternary troponin complex and the binary TnC.TnI complex in the presence and absence of Ca2+. Cys133 of TnI was chosen because our previous work has shown that the region of TnI containing this residue undergoes Ca2+-dependent movements between actin and TnC, and may play an important role in the regulatory function of troponin. For this purpose, a TnI mutant with a single Cys at position 133, and TnC mutants, each with a single Cys at positions 5, 12, 21, 41, 49, 89, 98, 133, and 158, were constructed by site-directed mutagenesis. The distances between TnI Cys133 and each of the nine residues in TnC were then measured. Using a least-squares minimization procedure, we determined the position of TnI Cys133 in the coordinate system of the crystal structure of TnC. Our results show that in the presence of Ca2+, TnI Cys133 is located near residue 12 beneath the N-terminal lobe of TnC, and moves away by 12.6 A upon the removal of Ca2+. TnI Cys133 and the region of TnC that undergoes major change in conformation in response to Ca2+ are located roughly on opposite sides of TnC's central helix. This suggests that the region in TnI that undergoes Ca2+-dependent interaction with TnC is distinct from that interacting with actin.

The C terminus of cardiac troponin I is essential for full inhibitory activity and Ca2+ sensitivity of rat myofibrils

Although the C terminus of troponin I is known to be important in myofilament Ca2+ regulation in skeletal muscle, the regulatory function of this region of cardiac troponin I (cTnI) has not been defined. To address this question, the following recombinant proteins were expressed in Escherichia coli and purified: mouse wild-type cTnI (WT cTnI; 211 residues), cTnI-(1-199) (missing 12 residues), cTnI-(1-188) (missing 23 residues), and cTnI-(1-151) (missing 60 residues). The inhibitory activity of cTnI and the mutants was tested in myofibrils, from which cTnI.cTnC was extracted by exchanging endogenous cardiac troponin with exogenous cTnT causing the Ca2+ sensitivity of the myofibrils to be lost. Addition of increasing amounts of exogenous WT cTnI or cTnI-(1-199) to cTnT-treated myofibrils at pCa 8 caused a concentration-dependent inhibition of the maximum ATPase activity. However, cTnI-(1-188) and cTnI-(1-151) inhibited this activity to about 75% and 50% of that of the WT cTnI, respectively. We also formed a complex of either WT cTnI or each of the mutants with cTnC, reconstituted the complex into the cTnT-treated myofibrils, and measured the Mg2+-ATPase activity as a function of pCa. We found that the cTnI-(1-188).cTnC complex only partially restored Ca2+ sensitivity, whereas the cTnI-(1-151).cTnC complex did not restore any Ca2+ sensitivity. Each cTnI C-terminal deletion mutant was able to bind to cTnC, as shown by urea-polyacrylamide gel-shift analysis and size exclusion chromatography. Each mutant also co-sedimented with actin. Our results indicate that residues 152-199 (C-terminal to the inhibitory region) of cTnI are essential for full inhibitory activity and Ca2+ sensitivity of myofibrillar ATPase activity in the heart.

Structural and functional domains of the troponin complex revealed by limited digestion

Troponin (Tn), consisting of three subunits, TnT, TnC, and TnI, plays a crucial role in the calcium-dependent regulation of vertebrate striated muscle contraction. In the present study, we have applied limited proteolysis to the Tn complex in order to study domain structures and to detect conformational differences of Tn under different conditions. We found that both TnT and TnI were susceptible to chymotryptic digestion: while TnT was cleaved into TnT-(1-158)-peptide and TnT-(159-259)-peptide irrespective of Ca2+ concentration, the cleavage sites of TnI were dependent on the Ca2+ occupancy of TnC. In addition, we characterized the effects of depletion of the C-terminal part of TnI on acto-S1 ATPase activity. The TnT-(159-259)-peptide-TnC-TnICa-frag complex [TnICa-frag = (TnI-(1-134 and 1-140)-peptide], which was produced in the presence of CaCl2 and MgCl2, retains both the activating and inhibitory capabilities of whole Tn on the acto-S1 ATPase activity, while TnT-(159-259)-peptide-TnC-TnIMg-frag complex [TnIMg-frag = (TnI-(1-116)-peptide], which was obtained in the presence of MgCl2 and EGTA, lost its ability to activate acto-S1 ATPase activity. Our results indicate that residues 117-134 or 117-140 of TnI undergo structural changes upon Ca(2+)-binding to the regulatory sites of TnC and are necessary for the Ca(2+)-dependent inhibitory action of the Tn complex on acto-S1 ATPase activity. We also showed that residues 135-181 or 141-181 of TnI are involved in the interaction of Tn with the tropomyosin-actin filament.

Production, crystallization, and preliminary X-ray analysis of rabbit skeletal muscle troponin complex consisting of troponin C and fragment (1-47) of troponin I

Troponin is a ternary protein complex consisting of subunits TnC. TnI, and TnT, and plays a key role in calcium regulation of the skeletal and cardiac muscle contraction. In the present study, a partial complex (CI47) was prepared from Escherichia coli-expressed rabbit skeletal muscle TnC and fragment 1-47 of TnI, which is obtained by chemical cleavage of an E. coli-expressed mutant of rabbit skeletal muscle TnI. Within the ternary troponin complex, CI47 is thought to form a core that is resistant to proteolytic digestion, and the interaction within CI47 likely maintains the integrity of the troponin complex. Complex CI47 was crystallized in the presence of sodium citrate. The addition of trehalose improved the diffraction pattern of the crystals substantially. The crystal lattice belongs to the space group P3(1)(2)21, with unit cell dimensions a = b = 48.2 A, c = 162 A. The asymmetric unit presumably contains one CI47 complex. Soaking with p-chloromercuribenzenesulfonate (PCMBS) resulted in loss of isomorphism, but enhanced the quality of the crystals. The crystals diffracted up to 2.3 A resolution, with completeness of 91% and R(merge) = 6.4%. The crystals of PCMBS-derivative should be suitable for X-ray studies using the multiple-wavelength anomalous diffraction technique. This is the first step for elucidating the structure of the full troponin complex.

Interaction of a troponin I inhibitory peptide with both domains of troponin C

Skeletal muscle contraction is regulated by Ca2+ binding to troponin (Tn), a complex of three proteins attached to the actin-tropomyosin filaments. We have been investigating key interactions of the Ca(2+)-binding protein TnC and the inhibitory protein TnI. Previously, we used 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) to produce zero-length cross-links in the complex of rabbit skeletal muscle TnC and TnI, and found that the N-terminal, regulatory domain of TnC formed cross-links to the inhibitory region of TnI (Leszyk, J., Grabarek, Z., Gergely, J. and Collins, J.H. (1990) Biochemistry 29, 299-304). In the present study we have used EDC to form cross-links between TnC and a synthetic peptide, based on residues 104-115 of TnI, which mimics intact TnI in its ability to inhibit actomyosin ATPase activity. Prior to cross-linking, we acetylated the epsilon-amino groups of the nine lysine residues of TnC in order to prevent intramolecular cross-linking. Cross-linked TnC-peptide products were cleaved with CNBr and several proteinases. The resulting cross-linked peptides were purified by HPLC and characterized by amino-acid sequence analysis. Our results indicate that the TnI peptide interacted most strongly with two sites in TnC: Glu-60 and/or Glu-61 in the N-terminal domain, and acidic residue(s) in segment 84-94 of the linker region which connects the N- and C-terminal domains of TnC.

Calcium-induced troponin flexibility revealed by distance distribution measurements between engineered sites

The contraction of vertebrate striated muscle is regulated by Ca2+ binding to troponin C (TnC). This causes conformational changes which alter the interaction of TnC with the inhibitory protein TnI and the tropomyosin-binding protein TnT. We have used the frequency domain method of fluorescence resonance energy transfer to measure TnT-TnC and TnT-TnI distances and distance distributions, in the presence of Ca2+, Mg2+, or EGTA, in TnC.TnI.TnT complexes. We reconstituted functional, ternary troponin complexes using the following recombinant subunits whose sequences were based on those of rabbit skeletal muscle: wild-type TnC; TnT25, a mutant C-terminal 25-kDa fragment of TnT containing a single Trp212 which was used as the sole donor for fluorescence energy transfer measurements; Trp-less TnI mutants which contained either no Cys or a single Cys at position 9, 96, or 117. Energy acceptor groups were introduced into TnC or TnI by labeling with dansyl aziridine or N-(iodoacetyl)-N'-(1-sulfo-5-naphthyl)ethylenediamine. Our results indicate that the troponin complex is relatively rigid in relaxed muscle, but becomes much more flexible when Ca2+ binds to regulatory sites in TnC. This increased flexibility may be propagated to the whole thin filament, releasing the inhibition of actomyosin ATPase activity and allowing the muscle to contract. This is the first report of distance distribution measurements between troponin subunits.

An NMR and spin label study of the effects of binding calcium and troponin I inhibitory peptide to cardiac troponin C

The paramagnetic relaxation reagent, 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy (HyTEMPO), was used to probe the surface exposure of methionine residues of recombinant cardiac troponin C (cTnC) in the absence and presence of Ca2+ at the regulatory site (site II), as well as in the presence of the troponin I inhibitory peptide (cTnIp). Methyl resonances of the 10 Met residues of cTnC were chosen as spectral probes because they are thought to play a role in both formation of the N-terminal hydrophobic pocket and in the binding of cTnIp. Proton longitudinal relaxation rates (R1's) of the [13C-methyl] groups in [13C-methyl]Met-labeled cTnC(C35S) were determined using a T1 two-dimensional heteronuclear single- and multiple-quantum coherence pulse sequence. Solvent-exposed Met residues exhibit increased relaxation rates from the paramagnetic effect of HyTEMPO. Relaxation rates in 2Ca(2+)-loaded and Ca(2+)-saturated cTnC, both in the presence and absence of HyTEMPO, permitted the topological mapping of the conformational changes induced by the binding of Ca2+ to site II, the site responsible for triggering muscle contraction. Calcium binding at site II resulted in an increased exposure of Met residues 45 and 81 to the soluble spin label HyTEMPO. This result is consistent with an opening of the hydrophobic pocket in the N-terminal domain of cTnC upon binding Ca2+ at site II. The binding of the inhibitory peptide cTnIp, corresponding to Asn 129 through Ile 149 of cTnI, to both 2Ca(2+)-loaded and Ca(2+)-saturated cTnC was shown to protect Met residues 120 and 157 from HyTEMPO as determined by a decrease in their measured R1 values. These results suggest that in both the 2Ca(2+)-loaded and Ca(2+)-saturated forms of cTnC, cTnIp binds primarily to the C-terminal domain of cTnC.

A model structure of the muscle protein complex 4Ca2+.troponin C.troponin I derived from small-angle scattering data: implications for regulation

We report here a model structure for 4Ca2+.troponin C.troponin I derived from small-angle X-ray and neutron scattering data using a Monte Carlo modeling method. In this model, troponin I appears as a spiral structure that wraps around 4Ca2+.troponin C which adopts an extended dumbbell conformation similar to that observed in the crystal structures of troponin C. The troponin I spiral has the approximate dimensions of an alpha-helix and winds through the hydrophobic "cups" in each globular domain of troponin C. The model is consistent with a body of previously published biochemical data on the interactions between troponin C and troponin I, and suggests the molecular mechanism for the Ca(2+)-sensitive switch that regulates the muscle contraction/relaxation cycle involves a signal transmitted via the central spiral region of troponin I.

The role of glycine (residue 89) in the central helix of EF-hand protein troponin-C exposed following amino-terminal alpha-helix deletion

Because an N-terminal alpha-helical (N-helix) arm and a KGK-triplet (residues 88KGK90) in the central helix of troponin-C (TnC) are missing in calmodulin, several recent studies have attempted to elucidate the structure-function correlations of these units. Presently, with a family of genetically manipulated derivatives especially developed for this study and tested on permeabilized isolated single skeletal muscle fiber segments, we explored the specificities of the amino acid residues within the N-helix and the KGK-triplet in TnC. Noticeably, the amino acid compositions vary between the N-helices of the cardiac and skeletal TnC isoforms. On the other hand, the KGK-triplet is located similarly in both TnC isoforms. We previously indicated that deletion of the N-helix (mutant delta Nt) diminishes the tension obtained on activation with maximal calcium, but the contractile function is revived by the superimposed deletion of the 88KGK90-triplet (mutant delta Nt delta KGK; see Gulati J, Babu A, Su H, Zhang YF, 1993, J Biol Chem 268:11685-11690). Using this functional test, we find that replacement of Gly-89 with a Leu or an Ala could also overcome the contractile defect associated with N-helix deletion. On the other hand, replacement of the skeletal TnC N-helix with cardiac type N-helix was unable to restore contractile function. The findings indicate a destabilizing influence of Gly-89 residue in skeletal TnC and suggest that the N-terminal arm in normal TnC serves to moderate this effect. Moreover, specificity of the N-helix between cardiac and skeletal TnCs raises the possibility that resultant structural disparities are also important for the functional distinctions of the TnC isoforms.