Polarized fluorescence depletion reports orientation distribution and rotational dynamics of muscle cross-bridges

Polarized two-photon photoselection in EGFP: Theory and experiment

In this work, we present a complete theoretical description of the excited state order created by two-photon photoselection from an isotropic ground state; this encompasses both the conventionally measured quadrupolar (K = 2) and the "hidden" degree of hexadecapolar (K = 4) transition dipole alignment, their dependence on the two-photon transition tensor and emission transition dipole moment orientation. Linearly and circularly polarized two-photon absorption (TPA) and time-resolved single- and two-photon fluorescence anisotropy measurements are used to determine the structure of the transition tensor in the deprotonated form of enhanced green fluorescent protein. For excitation wavelengths between 800 nm and 900 nm, TPA is best described by a single element, almost completely diagonal, two-dimensional (planar) transition tensor whose principal axis is collinear to that of the single-photon S₀ → S₁ transition moment. These observations are in accordance with assignments of the near-infrared two-photon absorption band in fluorescent proteins to a vibronically enhanced S₀ → S₁ transition.

Toward protein structure in situ: comparison of two bifunctional rhodamine adducts of troponin C

As part of a program to develop methods for determining protein structure in situ, sTnC was labeled with a bifunctional rhodamine (BR or BSR), cross-linking residues 56 and 63 of its C-helix. NMR spectroscopy of the N-terminal domain of BSR-labeled sTnC in complex with Ca(2+) and the troponin I switch peptide (residues 115-131) showed that BSR labeling does not significantly affect the secondary structure of the protein or its dynamics in solution. BR-labeling was previously shown to have no effect on the solution structure of this complex. Isometric force generation in isolated demembranated fibers from rabbit psoas muscle into which BR- or BSR-labeled sTnC had been exchanged showed reduced Ca(2+)-sensitivity, and this effect was larger with the BSR label. The orientation of rhodamine dipoles with respect to the fiber axis was determined by polarized fluorescence. The mean orientations of the BR and BSR dipoles were almost identical in relaxed muscle, suggesting that both probes accurately report the orientation of the C-helix to which they are attached. The BSR dipole had smaller orientational dispersion, consistent with less flexible linkers between the rhodamine dipole and cysteine-reactive groups.

Structural changes in troponin in response to Ca2+ and myosin binding to thin filaments during activation of skeletal muscle

Contraction of skeletal and cardiac muscle is regulated by Ca2+ -dependent structural changes in troponin that control the interaction between myosin and actin. We measured the orientations of troponin domains in skeletal muscle fibers using polarized fluorescence from bifunctional rhodamine probes on the C and E helices of troponin C. The C helix, in the regulatory head domain, tilts by approximately 30 degrees when muscle is activated in physiological conditions, with a Ca2+ -sensitivity similar to that of active force. Complete inhibition of active force did not affect C-helix orientation, and binding of rigor myosin heads did not affect its orientation at saturating [Ca2+]. The E helix, in the IT arm of troponin, tilted by approximately 10 degrees on activation, and this was reduced to only 3 degrees when active force was inhibited. Binding of rigor myosin heads produced a larger tilt of the E helix. Thus, in situ, the regulatory head acts as a pure Ca2+ -sensor, whereas the IT arm is primarily sensitive to myosin head binding. The polarized fluorescence data from active muscle are consistent with an in vitro structure of the troponin core complex in which the D and E helices of troponin C are collinear. The present data were used to orient this structure in the fiber and suggest that the IT arm is at approximately 30 degrees to the filament axis in active muscle. In relaxed muscle, the IT arm tilts to approximately 40 degrees but the D/E helix linker melts, allowing the regulatory head to tilt through a larger angle.

Kinetics of cardiac thin-filament activation probed by fluorescence polarization of rhodamine-labeled troponin C in skinned guinea pig trabeculae

A genetically engineered cardiac TnC mutant labeled at Cys-84 with tetramethylrhodamine-5-iodoacetamide dihydroiodide was passively exchanged for the endogenous form in skinned guinea pig trabeculae. The extent of exchange averaged nearly 70%, quantified by protein microarray of individual trabeculae. The uniformity of its distribution was verified by confocal microscopy. Fluorescence polarization, giving probe angle and its dispersion relative to the fiber long axis, was monitored simultaneously with isometric tension. Probe angle reflects underlying cTnC orientation. In steady-state experiments, rigor cross-bridges and Ca2+ with vanadate to inhibit cross-bridge formation produce a similar change in probe orientation as that observed with cycling cross-bridges (no Vi). Changes in probe angle were found at [Ca2+] well below those required to generate tension. Cross-bridges increased the Ca2+ dependence of angle change (cooperativity). Strong cross-bridge formation enhanced Ca2+ sensitivity and was required for full change in probe position. At submaximal [Ca2+], the thin filament regulatory system may act in a coordinated fashion, with the probe orientation of Ca2+-bound cTnC significantly affected by Ca2+ binding at neighboring regulatory units. The time course of the probe angle change and tension after photolytic release [Ca2+] by laser photolysis of NP-EGTA was Ca2+ sensitive and biphasic: a rapid component approximately 10 times faster than that of tension and a slower rate similar to that of tension. The fast component likely represents steps closely associated with Ca2+ binding to site II of cTnC, whereas the slow component may arise from cross-bridge feedback. These results suggest that the thin filament activation rate does not limit the tension time course in cardiac muscle.

Orientation of the myosin light chain region by single molecule total internal reflection fluorescence polarization microscopy

To study the orientation and dynamics of myosin, we measured fluorescence polarization of single molecules and ensembles of myosin decorating actin filaments. Engineered chicken gizzard regulatory light chain (RLC), labeled with bisiodoacetamidorhodamine at cysteine residues 100 and 108 or 104 and 115, was exchanged for endogenous RLC in rabbit skeletal muscle HMM or S1. AEDANS-labeled actin, fully decorated with labeled myosin fragment or a ratio of approximately 1:1000 labeled:unlabeled myosin fragment, was adhered to a quartz slide. Eight polarized fluorescence intensities were combined with the actin orientation from the AEDANS fluorescence to determine the axial angle (relative to actin), the azimuthal angle (around actin), and RLC mobility on the <<10 ms timescale. Order parameters of the orientation distributions from heavily labeled filaments agree well with comparable measurements in muscle fibers, verifying the technique. Experiments with HMM provide sufficient angular resolution to detect two orientations corresponding to the two heads in rigor. Experiments with S1 show a single orientation intermediate to the two seen for HMM. The angles measured for HMM are consistent with heads bound on adjacent actin monomers of a filament, under strain, similar to predictions based on ensemble measurements made on muscle fibers with electron microscopy and spectroscopic experiments.

Measurement of single macromolecule orientation by total internal reflection fluorescence polarization microscopy

A new approach is presented for measuring the three-dimensional orientation of individual macromolecules using single molecule fluorescence polarization (SMFP) microscopy. The technique uses the unique polarizations of evanescent waves generated by total internal reflection to excite the dipole moment of individual fluorophores. To evaluate the new SMFP technique, single molecule orientation measurements from sparsely labeled F-actin are compared to ensemble-averaged orientation data from similarly prepared densely labeled F-actin. Standard deviations of the SMFP measurements taken at 40 ms time intervals indicate that the uncertainty for individual measurements of axial and azimuthal angles is approximately 10 degrees at 40 ms time resolution. Comparison with ensemble data shows there are no substantial systematic errors associated with the single molecule measurements. In addition to evaluating the technique, the data also provide a new measurement of the torsional rigidity of F-actin. These measurements support the smaller of two values of the torsional rigidity of F-actin previously reported.

Bifunctional rhodamine probes of Myosin regulatory light chain orientation in relaxed skeletal muscle fibers

The orientation of the regulatory light chain (RLC) region of the myosin heads in relaxed skinned fibers from rabbit psoas muscle was investigated by polarized fluorescence from bifunctional rhodamine (BR) probes cross-linking pairs of cysteine residues introduced into the RLC. Pure 1:1 BR-RLC complexes were exchanged into single muscle fibers in EDTA rigor solution for 30 min at 30 degrees C; approximately 60% of the native RLC was removed and stoichiometrically replaced by BR-RLC, and >85% of the BR-RLC was located in the sarcomeric A-bands. The second- and fourth-rank order parameters of the orientation distributions of BR dipoles linking RLC cysteine pairs 100-108, 100-113, 108-113, and 104-115 were calculated from polarized fluorescence intensities, and used to determine the smoothest RLC orientation distribution-the maximum entropy distribution-consistent with the polarized fluorescence data. Maximum entropy distributions in relaxed muscle were relatively broad. At the peak of the distribution, the "lever" axis, linking Cys707 and Lys843 of the myosin heavy chain, was at 70-80 degrees to the fiber axis, and the "hook" helix (Pro830-Lys843) was almost coplanar with the fiber and lever axes. The temperature and ionic strength of the relaxing solution had small but reproducible effects on the orientation of the RLC region.

Myosin regulatory domain orientation in skeletal muscle fibers: application of novel electron paramagnetic resonance spectral decomposition and molecular modeling methods

Reorientation of the regulatory domain of the myosin head is a feature of all current models of force generation in muscle. We have determined the orientation of the myosin regulatory light chain (RLC) using a spin-label bound rigidly and stereospecifically to the single Cys-154 of a mutant skeletal isoform. Labeled RLC was reconstituted into skeletal muscle fibers using a modified method that results in near-stoichiometric levels of RLC and fully functional muscle. Complex electron paramagnetic resonance spectra obtained in rigor necessitated the development of a novel decomposition technique. The strength of this method is that no specific model for a complex orientational distribution was presumed. The global analysis of a series of spectra, from fibers tilted with respect to the magnetic field, revealed two populations: one well-ordered (+/-15 degrees ) with the spin-label z axis parallel to actin, and a second population with a large distribution (+/-60 degrees ). A lack of order in relaxed or nonoverlap fibers demonstrated that regulatory domain ordering was defined by interaction with actin rather than the thick filament surface. No order was observed in the regulatory domain during isometric contraction, consistent with the substantial reorientation that occurs during force generation. For the first time, spin-label orientation has been interpreted in terms of the orientation of a labeled domain. A Monte Carlo conformational search technique was used to determine the orientation of the spin-label with respect to the protein. This in turn allows determination of the absolute orientation of the regulatory domain with respect to the actin axis. The comparison with the electron microscopy reconstructions verified the accuracy of the method; the electron paramagnetic resonance determined that axial orientation was within 10 degrees of the electron microscopy model.

In Situ Orientations of Protein Domains

A recently developed approach for mapping protein-domain orientations in the cellular environment was used to investigate the Ca(2+)-dependent structural changes in the tropomyosin/troponin complex on the actin filament that regulate muscle contraction. Polarized fluorescence from bifunctional rhodamine probes attached along four alpha helices of troponin C (TnC) was measured in permeabilized skeletal muscle fibers. In relaxed muscle, the N-terminal lobe of TnC is less closed than in crystal structures of the Ca(2+)-free domain, and its D helix is approximately perpendicular to the actin filament. In contrast to crystal structures of isolated TnC, the D and E helices are not collinear. On muscle activation, the N lobe orientation becomes more disordered and the average angle between the C helix and the filament changes by 32 degrees +/- 5 degrees. These results illustrate the potential of in situ measurements of helix and domain orientations for elucidating structure-function relations in native macromolecular complexes.