Calmodulin supports the force-generating function in desensitized muscle fibers

Crystal Structures of a Poplar Thioredoxin Peroxidase that Exhibits the Structure of Glutathione Peroxidases: Insights into Redox-driven Conformational Changes

Glutathione peroxidases (GPXs) are a group of enzymes that regulate the levels of reactive oxygen species in cells and tissues, and protect them against oxidative damage. Contrary to most of their counterparts in animal cells, the higher plant GPX homologues identified so far possess cysteine instead of selenocysteine in their active site. Interestingly, the plant GPXs are not dependent on glutathione but rather on thioredoxin as their in vitro electron donor. We have determined the crystal structures of the reduced and oxidized form of Populus trichocarpaxdeltoides GPX5 (PtGPX5), using a selenomethionine derivative. PtGPX5 exhibits an overall structure similar to that of the known animal GPXs. PtGPX5 crystallized in the assumed physiological dimeric form, displaying a pseudo ten-stranded beta sheet core. Comparison of both redox structures indicates that a drastic conformational change is necessary to bring the two distant cysteine residues together to form an intramolecular disulfide bond. In addition, a computer model of a complex of PtGPX5 and its in vitro recycling partner thioredoxin h1 is proposed on the basis of the crystal packing of the oxidized form enzyme. A possible role of PtGPX5 as a heavy-metal sink is also discussed.

The functional role of the domains of troponin-C investigated with thrombin fragments of troponin-C reconstituted into skinned muscle fibers

Proteolysis of rabbit fast skeletal troponin-C (RSTnC) with thrombin produces four separate fragments containing the following Ca2+-binding site(s): TH1 (residues 1-120) sites I-III; TH2 (121-159) site IV; TH3 (1-100) sites I and II; and TH4 (101-120) site III. We studied the ability of these fragments to restore the steady state isometric force in TnC-depleted skinned skeletal muscle fibers. Interestingly, we found that all investigated fragments of RSTnC possessed some of the properties of native RSTnC, but none of them could fully regulate contraction in the fibers like intact RSTnC. TH1 was the most effective in the force restoration (65%) whereas the smaller fragments developed about 50% (TH3 and TH4) or 20% (TH2) of the initial force of unextracted fibers. Additionally, much higher concentrations of TH2, TH3, and TH4 compared to RSTnC OR TH1 were necessary for force development suggesting a decreased affinity of these fragments to their binding site(s) in the fibers. Like intact RSTnC, TH1 was able to interact with the fibers in a Ca(2+)-independent (Mg(2+)-dependent) manner, indicating that at a minimum, Ca(2+)-binding site III is required for this type of binding. The initial binding of the other fragments to the TnC-depleted fibers occurred only in the presence of Ca2+. TH2 and TH4 appeared to bind to two different binding sites in the fibers. The binding to one of the sites caused partial force restoration. This binding of TH2 and TH4 was abolished when Ca2+ was removed. TH2 and TH4 binding to the second site required Ca2+ initially but was maintained in the presence of Mg2+. This interaction of TH2 and TH4 partially blocked the rebinding of RSTnC to the fibers. The latter results suggest that site III and IV in these small fragments, when removed from the constraints of the parent protein, may assume conformations that allow them to function, to a certain extent, like both the regulatory sites (I and II) and the Ca(2+)-Mg2+ sites(III and IV) of TnC.

Concerted action of the high affinity calcium binding sites in skeletal muscle troponin C

Mutants of each of the four divalent cation binding sites of chicken skeletal muscle troponin C (TnC) were constructed using site-directed mutagenesis to convert Asp to Ala at the first coordinating position in each site. With a view to evaluating the importance of site-site interactions both within and between the N- and C-terminal domains, in this study the mutants are examined for their ability to associate with other components of the troponin-tropomyosin regulatory complex and to regulate thin filaments. The functional effects of each mutation in reconstitution assays are largely confined to the domain in which it occurs, where the unmutated site is unable to compensate for the defect. Thus the mutants of sites I and II bind to the regulatory complex but are impaired in ability to regulate tension and actomyosin ATPase activity, whereas the mutants of sites III and IV regulate activity but are unable to remain bound to thin filaments unless Ca2+ is present. When all four sites are intact, free Mg2+ causes a 50-60-fold increase in TnC's affinity for the other components of the regulatory complex, allowing it to attach firmly to thin filaments. Calcium can replace Mg2+ at a concentration ratio of 1:5000, and at this ratio the Ca2.TnC complex is more tightly bound to the filaments than the Mg2.TnC form. In the C-terminal mutants, higher concentrations of Ca2+ (above tension threshold) are required to effect this transformation than in the recombinant wild-type protein, suggesting that the mutants reveal an attachment mediated by Ca2+ in the N-domain sites.

Calmodulin is intrinsically LESS effective than troponin C in activating skeletal muscle contraction

Calmodulin (CaM) and troponin C (TnC) are evolutionarily and structurally homologous, yet they are not functionally interchangeable. In particular, CaM cannot effectively substitute for TnC as an activator of skeletal muscle contraction. To determine if this is a consequence of CaM's weak association with troponin T and I or the result of a more fundamental mechanistic defect, we have used CaM and a CaM[TnC] chimera, CaM[3,4 TnC], that stably associates with the thin filament. Replacement of TnC with CaM or CaM[3,4 TnC] reveals that CaM-like molecules reduce the Ca(2+)-sensitivity and cooperativity of activation, as well as the maximal Ca(2+)-activated tension. These observations indicate that CaM-like molecules are unable to continuously maintain the activated state of the thin filament.

The presence but not the sequence of the N-terminal peptide in cardiac TnC is important for function

The most diverged region of the primary amino acid sequence between cardiac (cTnC) and fast skeletal troponin C is the N-terminal ten amino acids. We report here that major changes in the primary sequence of this region in cTnC had a minimal effect on the ability of the mutant proteins to recover maximal activity in TnC-extracted cardiac and fast skeletal muscle myofibrils. However, deletion of the N-terminal nine amino acids resulted in a 60% decrease in maximal Ca(2+)-dependent ATPase activity with only a small change in the pCa50 of activation. Deletion of the N-terminal peptide did not appear to appreciably affect the Ca(2+)-binding properties of cTnC, but it did alter the interaction with hydrophobic fluorescent probes. Thus, the presence but not the sequence, of the N-terminal extension is important for the maximal activity of cTnC. The N-terminal helix may function in a relatively non-specific manner to prevent unfavorable interactions between domains in cTnC or between cTnC and other troponin subunits.

Trifluoperazine inhibition of contraction in permeabilized skeletal, cardiac and smooth muscles

To gain insights into the mechanism of the central helix of calmodulin and troponin-C in the Ca2(+)-regulation of force development in striated and smooth muscles, the present study was made of the TFP induced inhibition of contraction, and of the uptake of these proteins by skinned fibers. Calmodulin was four-fold more sensitive to TFP than TnC, but the inhibition was found to be identical for skeletal and cardiac muscles despite the differences in their troponin-C isoforms. Also, the results were comparable between fast-twitch fiber, when calmodulin was exchanged for troponin-C to act on TnI, and smooth muscle, where calmodulin acts on myosin light chain kinase. These findings indicate that the inhibition of force by TFP is entirely due to its binding to the hydrophobic sites in the central helix. The uptakes of troponin-C and calmodulin were also different, and this is explained by a TFP-independent domain in troponin-C that binds TnI.

Modification of temperature dependence of myofilament Ca sensitivity by troponin C replacement

The Ca sensitivity of chemically skinned right ventricular trabeculae from the rat heart was determined at 22 and 8 degrees C. Endogenous troponin C (TnC) was then extracted with EDTA and replaced with either bovine cardiac TnC or rabbit fast-twitch skeletal TnC. The temperature dependence of myofilament Ca sensitivity was then reevaluated. Cooling native cardiac tissue from 22 to 8 degrees C reduced the pCa (-log10 [Ca2+]), generating half-maximal tension (K1/2) from 5.20 +/- 0.07 to 4.89 +/- 0.08 (SD, n = 14), and also reduced maximum Ca-activated force to 33 +/- 6% of its value at 22 degrees C. After extraction of endogenous TnC and reconstitution with cardiac TnC, cooling from 22 to 8 degrees C caused a similar shift in mean K1/2 from 4.93 +/- 0.08 to 4.69 +/- 0.06 (n = 7). When skeletal TnC was reconstituted into TnC-extracted ventricular fibers, cooling from 22 to 8 degrees C led to a much smaller mean shift in K1/2 from 4.88 +/- 0.07 to 4.78 +/- 0.04 (n = 7). The results show that the magnitude of the cooling-induced shift in myofilament Ca sensitivity observed in the native state (or after reconstitution with cardiac TnC) is significantly reduced if the fiber is reconstituted with skeletal TnC (P less than 0.001). This indicates that the temperature dependence of myofilament Ca sensitivity of cardiac muscle can be modified by incorporation of skeletal TnC. Thus Ca binding to TnC plays an important role in determining the temperature dependence of myofilament Ca sensitivity.

Characterization of the Ca2+-switch in skeletal and cardiac muscles

To determine the significance of the global structure of the regulatory proteins in the mechanism of the Ca2+-switch in cardiac and skeletal muscle contractions, the properties of a family of Ca2+-binding proteins with 4 or 3 EF-hand motifs have been studied with desensitized skinned fiber preparations. Proteins with 4 EF hands (such as troponins C - TnCs) are dumb-bell shaped, those with 3 EF hands (parvalbumin) being ellipsoidal. The number of active sites varied between four and two. We find that the ability to anchor in the fiber is limited to proteins with 4 EF hands and, at least, two active Ca2+-binding sites, one each in the N- and C-termini. The results suggest that the dumb-bell shaped global structure is critical for the switching action in muscular contraction, and a trigger site in the N-terminus and a structural site in the C-terminus need to be active in order to regulate contractility.