Can Förster resonance energy transfer measurements uniquely position troponin residues on the actin filament? A case study in multiple-acceptor FRET

C-Terminal Basic Region of Troponin T Alters the Ca2+-Dependent Changes in Troponin I Interactions

The C-terminal 14-16 residues of human troponin T are required for full inactivation, and they prevent full activation at saturating Ca²⁺. Basic residues within that C-terminal region of TnT are essential for its function, but the mechanism of action is unknown. That region of TnT is natively disordered and does not appear in reconstructions of the troponin structure. We used Förster resonance energy transfer to determine if the C-terminal basic region of TnT alters transitions of TnI or if it operates independently. We also examined Ca²⁺-dependent changes in the C-terminal region of TnT itself. Probes on TnI-143 (inhibitory region) and TnI-159 (switch region) moved away from sites on actin and tropomyosin and toward TnC-84 at high Ca²⁺. Ca²⁺ also displaced C-terminal TnT from actin-tropomyosin but without movement toward TnC. Deletion of C-terminal TnT produced changes in TnI-143 like those effected by Ca²⁺, but effects on TnI-159 were muted; there was no effect on the distance of the switch region to TnC-84. Substituting Ala for basic residues within C-terminal TnT displaced C-terminal TnT from actin-tropomyosin. The results suggest that C-terminal TnT stabilizes tropomyosin in the inactive position on actin. Removal of basic residues from C-terminal TnT produced a Ca²⁺-like state except that the switch region of TnI was not bound to TnC. Addition of Ca²⁺ caused more extreme displacement from actin-tropomyosin as the active state became more fully occupied as in the case of wild-type TnT in the presence of both Ca²⁺ and bound rigor myosin S1.

Structural studies of interactions between cardiac troponin I and actin in regulated thin filament using Förster resonance energy transfer

The Ca(2+)-induced interaction between cardiac troponin I (cTnI) and actin plays a key role in the regulation of cardiac muscle contraction and relaxation. In this report we have investigated changes of this interaction in response to strong cross-bridge formation between myosin S1 and actin and PKA phosphorylation of cTnI within reconstituted thin filament. The interaction was monitored by measuring Förster resonance energy transfer (FRET) between the fluorescent donor 5-(iodoacetamidoethyl)aminonaphthalene-1-sulfonic acid (AEDANS) attached to the residues 131, 151, 160 167, 188, and 210 of cTnI and the nonfluorescent acceptor 4-(dimethylamino)phenylazophenyl-4'-maleimide (DABM) attached to cysteine 374 of actin. The FRET distance measurements showed that bound Ca(2+) induced large increases in the distances from actin to the cTnI sites, indicating a Ca(2+)-triggered separation of cTnI from actin. Strongly bound myosin S1 induced additional increases in these distances in the presence of bound Ca(2+). The two ligand-induced increases were independent of each other. These two-step changes in distances provide a direct link of structural changes at the interface between cTnI and actin to the three-state model of thin filament regulation of muscle contraction and relaxation. When cTnC was inactivated through mutations of key residues within the 12-residue Ca(2+)-binding loop, strongly bound S1 alone induced increases in the distances in spite of the fact that the filaments no longer bound regulatory Ca(2+). These results suggest bound Ca(2+) or strongly bound S1 alone can partially activate thin filament, but full activation requires both bound Ca(2+) and strongly bound S1. The distributions of the FRET distances revealed different structural dynamics associated with different regions of cTnI in different biochemical states. The second actin-binding region appears more rigid than the inhibitory/regulatory region. In the Mg(2+) state, the regulatory region appears more flexible than the inhibitory region, and in the Ca(2+) state the inhibitory region becomes more flexible. PKA phosphorylation of cTnI at Ser23 and Ser24 distance from actin to cTnI residue 131 by 2.2-5.2 A in different biochemical states and narrowed the distributions of the distances from actin to the inhibitory and regulatory regions of cTnI. The observed phosphorylation effects are likely due to an intramolecular interaction of the phosphorylated N-terminal segment and the inhibitory region of cTnI.

The cardiac Ca2+-sensitive regulatory switch, a system in dynamic equilibrium

The Ca(2+)-sensitive regulatory switch of cardiac muscle is a paradigmatic example of protein assemblies that communicate ligand binding through allosteric change. The switch is a dimeric complex of troponin C (TnC), an allosteric sensor for Ca(2+), and troponin I (TnI), an allosteric reporter. Time-resolved equilibrium Förster resonance energy transfer (FRET) measurements suggest that the switch activates in two steps: a TnI-independent Ca(2+)-priming step followed by TnI-dependent opening. To resolve the mechanistic role of TnI in activation we performed stopped-flow FRET measurements of activation after rapid addition of a lacking component (Ca(2+) or TnI) and deactivation after rapid chelation of Ca(2+). Time-resolved measurements, stopped-flow measurements, and Ca(2+)-titration measurements were globally analyzed in terms of a new quantitative dynamic model of TnC-TnI allostery. The analysis provided a mesoscopic parameterization of distance changes, free energy changes, and transition rates among the accessible coarse-grained states of the system. The results reveal that 1), the Ca(2+)-induced priming step, which precedes opening, is the rate-limiting step in activation; 2), closing is the rate-limiting step in de-activation; 3), TnI induces opening; 4), there is an incompletely deactivated population when regulatory Ca(2+) is not bound, which generates an accessory pathway of activation; and 5), there is incomplete activation by Ca(2+)-when regulatory Ca(2+) is bound, a 3:2 mixture of dynamically interconverting open (active) and primed-closed (partially active) conformers is observed (15 degrees C). Temperature-dependent stopped-flow FRET experiments provide a near complete thermokinetic parameterization of opening: the enthalpy change (DeltaH = -33.4 kJ/mol), entropy change (DeltaS = -0.110 kJ/mol/K), heat capacity change (DeltaC(p) = -7.6 kJ/mol/K), the enthalpy of activation (delta(double dagger) = 10.6 kJ/mol) and the effective barrier crossing attempt frequency (nu(adj) = 1.8 x 10(4) s(-1)).

Nuclear protein‐induced bending and flexing of the hypoxic response element of the rat vascular endothelial growth factor promoter

Bending and flexing of DNA may contribute to transcriptional regulation. Because hypoxia and other physiological signals induce formation of an abasic site at a key base within the hypoxic response element (HRE) of the vascular endothelial growth factor (VEGF) gene (FASEB J. 19, 387-394, 2005) and because abasic sites can introduce flexibility in model DNA sequences, in the present study we used a fluorescence resonance energy transfer-based reporter system to assess topological changes in a wild-type (WT) sequence of the HRE of the rat VEGF gene and in a sequence harboring a single abasic site mimicking the effect of hypoxia. Binding of the hypoxia-inducible transcriptional complex present in hypoxic pulmonary artery endothelial cell nuclear extract to the WT sequence failed to alter sequence topology whereas nuclear protein binding to the modified HRE engendered considerable sequence flexibility. Topological effects of nuclear proteins on the modified VEGF HRE were dependent on the transcription factor hypoxia-inducible factor-1 and on formation of a single-strand break at the abasic site mediated by the coactivator, Ref-1/Ape1. These observations suggest that oxidative base modifications in the VEGF HRE evoked by physiological signals could be a precursor to single-strand break formation that has an impact on gene expression by modulating sequence flexibility.

Oligomeric state and mode of self-association of Thermotoga maritima ribosomal stalk protein L12 in solution

The "stalk" of the prokaryotic 50S ribosomal subunit is comprised of four copies of the protein L7/L12. In Escherichia coli, L7/L12 is a dimeric protein at micromolar concentrations, which is able to undergo rapid subunit exchange. A recent structural study indicated a tetrameric arrangement of the L12 proteins isolated from Thermotoga maritima, in which the proteins engaged in two different dimerization modes. In one mode, the two monomers of L12 form a tight symmetric and parallel dimer held together by a four-helix bundle, which encompasses the hinge region between the N- and C-terminal domains. In the other mode, the two monomers bind through their N-terminal region in an antiparallel configuration, in which one monomer comprises an alpha-helical hinge and the other monomer adopts an elongated shape with an unfolded hinge region. Presently, it is unclear which dimer contact prevails in solution and on the ribosome. Using cysteine mutants of T. maritima labeled with fluorescent probes, we investigated the mode of interactions between L12 subunits. Data from Forster resonance energy transfer experiments support a dimerization of L12 in solution, in which two monomers bind through their N-terminal region in an antiparallel configuration. We also demonstrate that the rate of subunit exchange in T. maritima L12 is significantly slower at 25 degrees C than that in the E. coli system. The exchange rate increases with increasing temperature and approaches the one observed for the E. coli system at 50 degrees C. Possible factors responsible for this difference are discussed.

Structural based insights into the role of troponin in cardiac muscle pathophysiology

Troponin is a molecular switch, directly regulating the Ca2+-dependent activation of myofilament in striated muscle contraction. Cardiac troponin is subject to covalent and noncovalent modifications; phosphorylation modulates myofilament physiology, mutations are linked to familial hypertrophic cardiomyopathy, intracellular acidification causes myocardial infarction, and cardiotonic drugs modify myofilament response to Ca2+. The structure of troponin provides insights into the mechanism of this molecular switch and an understanding of the effects of protein modification under pathophysiological conditions. Although the structure of troponin C has been solved in various Ca2+-bound states for some time, structural information on troponin I and troponin T has only emerged recently. This review summarizes recent advances on the structure of complexes of troponin subunits with the aim of assessing how these proteins interact with each other to execute its role as a molecular switch and how covalent and noncovalent modifications affect the structure of troponin and the switch mechanism. We focus on pinpointing the specific amino acid residues involved in phosphorylation and mutation and the pH sensitive regions in the structure of troponin. We also present recent structural work that have identified the docking sites of several cardiotonic drugs on cardiac troponin C and discuss their relevance in the direction of troponin based drug design in the therapy of heart disease.

Switching of Troponin I: Ca2+ and Myosin-induced Activation of Heart Muscle

The principal task of the Ca(2+) activation of striated muscle is the release of the troponin I (TnI) inhibitory region (TnI-I) from actin. TnI-I release facilitates the repositioning of tropomyosin across the actin surface and the formation of strong, force generating, actin-myosin cross-bridges. Full activation of the Ca(2+) regulatory switch (CRS) requires two switching steps in cTnI: binding of the TnI regulatory region to hydrophobic sites in the N-domain of Ca(2+)-bound troponin C and release of the adjacent TnI-I from actin. Using Förster resonance energy transfer, we have examined the requirements for full activation of the cardiac CRS. In the presence of actin, both Ca(2+) and strong cross-bridges are required for full activation. Actin desensitizes the CRS to Ca(2+) and produces cooperativity in the Ca(2+) activation of the CRS. Strong cross-bridges eliminate cooperativity and re-sensitize the CRS to Ca(2+). We propose a kinetic scheme and a structural model to account for these findings.

A fluorescence resonance energy transfer-derived structure of a quantum dot-protein bioconjugate nanoassembly

The first generation of luminescent semiconductor quantum dot (QD)-based hybrid inorganic biomaterials and sensors is now being developed. It is crucial to understand how bioreceptors, especially proteins, interact with these inorganic nanomaterials. As a model system for study, we use Rhodamine red-labeled engineered variants of Escherichia coli maltose-binding protein (MBP) coordinated to the surface of 555-nm emitting CdSe-ZnS core-shell QDs. Fluorescence resonance energy transfer studies were performed to determine the distance from each of six unique MBP-Rhodamine red dye-acceptor locations to the center of the energy-donating QD. In a strategy analogous to a nanoscale global positioning system determination, we use the intraassembly distances determined from the fluorescence resonance energy transfer measurements, the MBP crystallographic coordinates, and a least-squares approach to determine the orientation of the MBP relative to the QD surface. Results indicate that MBP has a preferred orientation on the QD surface. The refined model is in agreement with other evidence, which indicates coordination of the protein to the QD occurs by means of its C-terminal pentahistidine tail, and the size of the QD estimated from the model is in good agreement with physical measurements of QD size. The approach detailed here may be useful in determining the orientation of proteins in other hybrid protein-nanoparticle materials. To our knowledge, this is the first structural model of a hybrid luminescent QD-protein receptor assembly elucidated by using spectroscopic measurements in conjunction with crystallographic and other data.