Regulation of troponin C synthesis in primary culture of chicken cardiac muscle cells

Cardiac myocyte cell culture from fourteen day old embryonic chicken heart was prepared. This cultured cell system was used to examine the regulation of troponin C (TnC) synthesis in cardiac muscle. To examine the regulation of TnC polypeptide synthesis, cardiac myocyte cells were pulse labelled with ³⁵S-methionine at different days after plating. The synthesis of TnC was measured by determining the amount of radioactivity incorporated into the TnC polypeptide following separation by two dimensional gel electrophoresis. These measurements showed that TnC synthesis was maximum in 36 to 48 h old cultures and reached its lowest level in 4 day old cultures. This was in contrast to the synthesis of actin and tropomyosin. Synthesis of these polypeptides were lowest in 36 to 48 h old cultures and was maximum in 7 day old cultures. To examine whether the synthesis of TnC polypeptide paralleled the levels of TnC mRNA the sequences homologous to quail slow TnC cDNA clone were measured by hybridisation. The results showed that the decrease in the synthesis of troponin C polypeptide cannot be fully explained by the decrease in the steady state level of troponin C mRNA. The possibility of a role of translational control of troponin C mRNA in this process is discussed.     

Fast and slow isoforms of troponin I and troponin C. Distribution in normal rabbit muscles and effects of chronic stimulation

Polyclonal antibodies were raised against troponin I (TnI) and troponin C (TnC) purified from fast-twitch and slow-twitch rabbit muscles. These antibodies were used to elucidate the distribution of fast and slow isoforms of TnI and TnC in normal and chronically stimulated rabbit hind limb muscles by immunoblots of one-dimensional and two-dimensional electrophoreses. In contrast to the multiplicity of fast and slow troponin T (TnT) isoforms, TnI and TnC were present as unique fast and slow isoforms. Whereas no charge variants were detected for slow TnI, fast TnI was present in at least three charge variants. As judged from the results of alkaline phosphatase digestion, these charge variants represent differently phosphorylated forms. Fast and slow TnC both exist as two charge variants which, however, were unaffected by alkaline phosphatase treatment. Chronic low-frequency stimulation of fast-twitch muscles induced progressive increases in the slow isoforms of TnC and TnI at the expense of their fast isoforms. The extent of the fast-to-slow transition was more pronounced in the case of TnC than in that of TnI. Long-term stimulated muscles with a complete fast-to-slow transition, at the level of the TnT isoforms, still contained fast and slow isoforms of both TnI and TnC. The coexistence of fast and slow isoforms of the three troponin subunits in the transforming muscle was interpreted as indicating the presence of hybrid troponin molecules composed of fast and slow isoforms. Studies at the mRNA level showed changes similar to those at the protein level. However, in long-term stimulated muscles, the fast-to-slow transition of TnI was more pronounced at the mRNA level than at the protein level.     

Perinuclear localization of slow troponin C m RNA in muscle cells is controlled by a cis-element located at its 3' untranslated region

The process of mRNA localization within a specific cytoplasmic region is an integral aspect of the regulation of gene expression. Furthermore, colocalization of mRNAs and their respective translation products may facilitate the proper assembly of multi-subunit complexes like the thick and thin filaments of muscle. This postulate was tested by investigating the cytoplasmic localization of three mRNAs-the alpha-actin, slow troponin C (sTnC), and slow troponin I (sTnI), which encode different poly-peptide partners of the thin filament. Using in situ hybridization we showed that all three thin filament mRNAs are localized in the perinuclear cytoplasm of cultured C2C12 muscle cells. Their localization differs from that of the nonmuscle beta-actin mRNA, which is localized in the peripheral region of both proliferating nondifferentiated myoblasts and the differentiated myocytes. Analysis of the localization signal of the sTnC mRNA showed that a 40-nucleotide-long region of the sTnC mRNA 3' UTR is sufficient to confer the perinuclear localization on a heterologous reporter beta-Gal mRNA. This localization signal showed tissue specificity and worked only in the differentiated myocytes, but not in the proliferating myoblasts or in HeLa cells. The predicted secondary structure of the localization signal suggests the presence of multiple stem and loop structures in this region of the 3' UTR. Mutations within the stem region of the localization signal, which abolish the base pairing in this region, significantly reduced its perinuclear mRNA localization activity. Using UV-induced photo-cross-linking of RNA and proteins we found that a myotube-specific 42-kDa polypeptide binds to the localization signal.     

Interaction of troponin C and troponin C fragments with troponin I and the troponin I inhibitory peptide.

We have quantitated the interactions of two rabbit skeletal troponin C fragments with troponin I and the troponin I inhibitory peptide. The calcium binding properties of the fragments and the ability of the fragments to exert control in the regulated actomyosin ATPase assay have also been studied. The N- and C-terminal divalent metal binding domains of rabbit skeletal troponin C, residues 1-97 and residues 98-159, respectively, were prepared by specific cleavage at cysteine-98 and separation by gel exclusion chromatography. Both of the troponin C fragments bind calcium. The calcium affinity of the weak sites within the N-terminal fragment is about an order of magnitude greater than is reported for these sites in troponin C, suggesting interaction between the calcium-saturated strong sites and the weak sites. Stoichiometric binding (1:1) of the troponin I inhibitory peptide to each fragment and to troponin C increased the calcium affinities of the fragments and troponin C. Complex formation was detected by fluorescence quenching or enhancement using dansyl-labeled troponin C (and fragments) or tryptophan-labeled troponin I inhibitory peptide. The troponin C fragments bind to troponin I with 1:1 stoichiometry and approximately equal affinities (1.6 x 10(6) M-1) which are decreased 4-fold in the presence of magnesium versus calcium. These calcium effects are much smaller than is observed for troponin C. The summed free energies for the binding of the troponin C fragments to troponin I are much larger than the free energy of binding troponin C. This suggests a large positive interaction free energy for troponin C binding to troponin I relative to the fragments.(ABSTRACT TRUNCATED AT 250 WORDS)     

Conformation of the regulatory domain of cardiac muscle troponin C in its complex with cardiac troponin I.

Calcium activation of fast striated muscle results from an opening of the regulatory N-terminal domain of fast skeletal troponin C (fsTnC), and a substantial exposure of a hydrophobic patch, essential for Ca(2+)-dependent interaction with fast skeletal troponin I (fsTnI). This interaction is obligatory to relieve the inhibition of strong, force-generating actin-myosin interactions. We have determined intersite distances in the N-terminal domain of cardiac TnC (cTnC) by fluorescence resonance energy transfer measurements and found negligible increases in these distances when the single regulatory site is saturated with Ca(2+). However, in the presence of bound cardiac TnI (cTnI), activator Ca(2+) induces significant increases in the distances and a substantial opening of the N-domain. This open conformation within the cTnC.cTnI complex has properties favorable for the Ca(2+)-induced interaction with an additional segment of cTnI. Thus, the binding of cTnI to cTnC is a prerequisite to achieve a Ca(2+)-induced open N-domain similar to that previously observed in fsTnC with no bound fsTnI. This role of cardiac TnI has not been previously recognized. Our results also indicate that structural information derived from a single protein may not be sufficient for inference of a structure/function relationship.     

Characterization of G protein-coupled receptor regulation in antisense mRNA-expressing cells with reduced arrestin levels.

Previous studies with overexpressing wild-type or dominant negative nonvisual arrestins have established a role for these proteins in beta2-adrenergic receptor (beta2AR) internalization, desensitization, and resensitization. To validate and extend such findings, we employed an antisense strategy to target the nonvisual arrestins, arrestin-2 and arrestin-3, and determined the associated effects on the regulation of G protein-coupled receptor (GPCR) signaling. HEK293 cells stably expressing antisense constructs targeting arrestin-2 exhibited a selective reduction (approximately 50%) in arrestin-2 levels, while arrestin-3 antisense constructs resulted in reductions (>/=50%) in both arrestin-2 and arrestin-3 levels. Initial analysis of these cells demonstrated that a reduced level of arrestin expression resulted in a significant decrease in the extent of agonist-induced internalization of exogenously expressed beta2ARs, but had no effect on internalization of either m2 or m3 muscarinic acetylcholine receptors. Additional characterization involved assessing the role of arrestins in the regulation of endogenous GPCRs in these cells. Reduced arrestin levels significantly decreased the rate of endogenous beta2AR internalization, desensitization, and resensitization. Further analysis demonstrated that the desensitization of endogenous A2b adenosine and prostaglandin E2-stimulated receptors was also attenuated in cells with reduced arrestin levels. The effects on the beta2-adrenergic, A2b adenosine, and PGE2-stimulated receptors were similar among cell lines that exhibited either a selective reduction in arrestin-2 levels or a reduction in both arrestin-2 and -3 levels. These findings establish the utility of antisense approaches in the examination of arrestin-mediated GPCR regulation.     

Slow troponin C is present in both muscle and nonmuscle cells.

A common primer was used for synthesis of cDNAs from both chicken fast and slow troponin C (TnC) mRNAs. Synthesis of double-stranded cDNAs and their amplification by polymerase chain reaction gave specific products corresponding to these mRNAs. This method was used for determining the presence of TnC mRNAs in various tissues. Our results show that while the fast TnC mRNA is expressed only in the muscle cells, slow TnC mRNA is expressed in a number of nonmuscle cells. Not all nonmuscle cells, however, express slow TnC mRNA. Liver and brain tissues showed the presence of high levels of this mRNA, while it was absent in chicken smooth muscle and embryonic skin. The slow TnC mRNA was very stable in cardiac muscle cells. It degrades with a half-life of approximately 94 h. The same mRNA was less stable in skeletal muscle and liver cells. The half-lives were found to be only between 13 and 16 h in these cells. Our results suggest that slow TnC mRNA may function as the nonmuscle isoform of this contractile protein. Since slow TnC mRNA is the only TnC isoform present in cardiac muscle, liver and brain, it is possible that besides its role in regulating contraction of striated muscle slow TnC can also function in processes other than muscle contraction.     

Effect of reduced temperatures on protein synthesis in mouse L cells.

The rate of incorporation of leucine into protein, the rate of polypeptide elongation and termination, and the relative quantity and size of polysomes were analyzed in mouse L cells grown in suspension culture at various temperatures between 0 degrees C and 36 degrees C. Between 10 degrees C and 36 degrees C protein synthesis exhibited two different apparent activation energies (39 kcal/mole, 10-25 degrees C; 14 kcal/mole, 25-36 degrees C), whereas elongation and termination had only one (16 kcal/mole). Below 36 degrees C, the polysome level and size decreased, reaching a minimum of 30% of the control 36 degrees C values at 10 degrees C; below 10 degrees C the level increased again back to control values at 0 degrees C. The polysome decline was time dependent, requiring about 5 hr to reach the equilibrium value. This decline is completely reversible within 60 min, even in the presence of 4 mug/ml of actinomycin D, and even after 15 hr of incubation at the lower temperature. The results suggest that polypeptide initiation is rate limiting, particularly below 25 degrees C; whereas above this temperature, elongation or perhaps some other process may be limiting. These results are quite different from those obtained for E. coli and rabbit reticulocyte protein synthesis.     

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.     

Distance distributions and anisotropy decays of troponin C and its complex with troponin I

We used frequency domain measurements of fluorescence resonance energy transfer to recover the distribution of distances between Met 25 and Cys 98 in rabbit skeletal troponin C. These residues were labeled with dansylaziridine as energy donor and 5-(iodoacetamido)eosin as acceptor and are located on the N- and C-terminal lobes of the two-domain protein, respectively. We developed a procedure to correct for the fraction of the sample that was incompletely labeled with the acceptor independent of chemical data. At pH 7.5 and in the presence of Mg2+, the mean distance was near 15 A with a half-width of the distribution of 15 A; when Mg2+ was replaced by Ca2+, the mean distance increased to 22 A with a decrease in the half-width by 4 A. Similar but less pronounced differences in the mean distance and half-width between samples containing Mg2+ and Ca2+ were also observed with troponin C complexed to troponin I. The results suggest that the conformation of troponin C is altered by Ca2+ binding to the Ca(2+)-specific sites and displacing bound Mg2+ at the Ca2+/Mg2+ sites. This alteration may play an important role in Ca2+ signaling in muscle. At pH 7.5, the anisotropy decays of the donor-labeled troponin C showed two components, with the long rotational correlation time (12 ns) reflecting the overall motion of the protein. When the pH was lowered from 7.5 to 5.2, the mean distribution distance of apotroponin C increased from 22 to 32 A and the half-width decreased by a factor of 2 from 13 to 7 A. The long correlation time of apotroponin C increased to 19 ns at the acidic pH. These results are discussed in terms of a model in which skeletal troponin C is a dimer at low pH and enable comparison of the solution conformation of the protein at neutral pH with a crystal structure obtained at pH 5.2. While the conformation of the monomeric unit of troponin C dimer at pH 5.2 is extended and consistent with the crystal structure, the conformation at neutral pH is likely more compact than the crystal structure predicts.     

Activation of nuclear polypeptide synthesis in rat liver cells after inhibition of template protein synthesis. The role of nuclear polypeptide synthesis in the changes of the chromatin structure

The correlation between the rates of nuclear polypeptide synthesis (NPS) and matrix protein synthesis in rat liver cells was investigated. It was shown that NPS is activated under conditions of protein synthesis inhibition in the cytoplasm. Model experiments revealed that the NPS and ADP ribosylation systems compete for chromatin structure: ADP ribosylation induces condensation, while NPS--decondensation of chromatin.     

Proximity relationships between residue 6 of troponin I and residues in troponin C: further evidence for extended conformation of troponin C in the troponin complex

Skeletal muscle troponin C (TnC) adopts an extended conformation when crystallized alone and a compact one when crystallized with an N-terminal troponin I (TnI) peptide, TnI(1-47) [Vassylyev et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 4847-4852]. The N-terminal region of TnI (residues 1-40) was suggested to play a functional role of facilitating the movement of TnI's inhibitory region between TnC and actin [Tripet et al. (1997) J. Mol. Biol. 271, 728-750]. To test this hypothesis and to investigate the conformation of TnC in the intact troponin complex and in solution, we attached fluorescence and photo-cross-linking probes to a mutant TnI with a single cysteine at residue 6. Distances from this residue to residues of TnC were measured by the fluorescence resonance energy transfer technique, and the sites of photo-cross-linking in TnC were determined by microsequencing and mass spectrometry following enzymatic digestions. Our results show that in the troponin complex neither the distance between TnI residue 6 and TnC residue 89 nor the photo-cross-linking site in TnC, Ser133, changes with Ca(2+), in support of the notion that this region plays mainly a structural rather than a regulatory role. The distances to residues 12 and 41 in TnC's N-domain are both considerably longer than those predicted by the crystal structure of TnC.TnI(1-47), supporting an extended rather than a compact conformation of TnC. In the binary TnC.TnI complex and the presence of Ca(2+), Met43 in TnC's N-domain was identified as the photo-cross-linking site, and multiple distances between TnI residue 6 and TnC residue 41 were detected. This was taken to indicate increased flexibility in TnC's central helix and that TnC dynamically changes between a compact and an extended conformation when troponin T (TnT) is absent. Our results further emphasize the difference between the binary TnC.TnI and the ternary troponin complexes and the importance of using intact proteins in the study of structure-function relationships of troponin.