Path and extent of cross-bridge rotation during muscle contraction

Myopathy-causing Q147P TPM2 mutation shifts tropomyosin strands further towards the open position and increases the proportion of strong-binding cross-bridges during the ATPase cycle

The molecular mechanisms of skeletal muscle dysfunction in congenital myopathies remain unclear. The present study examines the effect of a myopathy-causing mutation Q147P in β-tropomyosin on the position of tropomyosin on troponin-free filaments and on the actin–myosin interaction at different stages of the ATP hydrolysis cycle using the technique of polarized fluorimetry. Wild-type and Q147P recombinant tropomyosins, actin, and myosin subfragment-1 were modified by 5-IAF, 1,5-IAEDANS or FITC-phalloidin, and 1,5-IAEDANS, respectively, and incorporated into single ghost muscle fibers, containing predominantly actin filaments which were free of troponin and tropomyosin. Despite its reduced affinity for actin in co-sedimentation assay, the Q147P mutant incorporates into the muscle fiber. However, compared to wild-type tropomyosin, it locates closer to the center of the actin filament. The mutant tropomyosin increases the proportion of the strong-binding myosin heads and disrupts the co-operation of actin and myosin heads during the ATPase cycle. These changes are likely to underlie the contractile abnormalities caused by this mutation.

Mapping the orientation of nuclear pore proteins in living cells with polarized fluorescence microscopy

The nuclear pore complex (NPC) perforates the nuclear envelope to facilitate selective transport between nucleus and cytoplasm. The NPC is composed of multiple copies of ∼30 different proteins, termed nucleoporins, whose arrangement within the NPC is an important unsolved puzzle in structural biology. Various alternative models for NPC architecture have been proposed but not tested experimentally in intact NPCs. We present a method using polarized fluorescence microscopy to investigate nucleoporin orientation in live yeast and mammalian cells. Our results support an arrangement of both yeast Nic96 and human Nup133-Nup107 in which their long axes are approximately parallel to the nuclear envelope plane. The method we developed can complement X-ray crystallography and electron microscopy to generate a high-resolution map of the entire NPC, and may be able to monitor nucleoporin rearrangements during nucleocytoplasmic transport and NPC assembly. This strategy can also be adapted for other macromolecular machines.

Determination of orientation distribution function of anisotropic paramagnetic species by analysis of ESR spectra angular dependence

A method has been developed to determine orientation distribution function (ODF) of anisotropic paramagnetic species by analysis of the angular dependence of the ESR spectra. The method is based on computational spectra simulation. The ODF is represented as an expansion in terms of orthonormal functions. The expansion coefficients are determined through minimization of discrepancies between simulated spectra and experimental ones. By means of the suggested method we have determined the orientation distribution functions for radical probe 2,2,6,6-tetramethyl-4-ol-piperidinooxyl in 4-n-amyl-4'-cyanobiphenyl aligned by magnetic field and 2-septadecyl-2,3,4,5,5-pentamethylimidazolidine in polyethylene stretched films. In each case, thermal evolution of the ODF has been investigated.

A maximum entropy analysis of protein orientations using fluorescence polarization data from multiple probes

Techniques have recently become available to label protein subunits with fluorescent probes at predetermined orientation relative to the protein coordinates. The known local orientation enables quantitative interpretation of fluorescence polarization experiments in terms of orientation and motions of the protein within a larger macromolecular assembly. Combining data obtained from probes placed at several distinct orientations relative to the protein structure reveals functionally relevant information about the axial and azimuthal orientation of the labeled protein segment relative to its surroundings. Here we present an analytical method to determine the protein orientational distribution from such data. The method produces the broadest distribution compatible with the data by maximizing its informational entropy. The key advantages of this approach are that no a priori assumptions are required about the shape of the distribution and that a unique, exact fit to the data is obtained. The relative orientations of the probes used for the experiments have great influence on information content of the maximum entropy distribution. Therefore, the choice of probe orientations is crucial. In particular, the probes must access independent aspects of the protein orientation, and two-fold rotational symmetries must be avoided. For a set of probes, a "figure of merit" is proposed, based on the independence among the probe orientations. With simulated fluorescence polarization data, we tested the capacity of maximum entropy analysis to recover specific protein orientational distributions and found that it is capable of recovering orientational distributions with one and two peaks. The similarity between the maximum entropy distribution and the test distribution improves gradually as the number of independent probe orientations increases. As a practical example, ME distributions were determined with experimental data from muscle fibers labeled with bifunctional rhodamine at known orientations with respect to the myosin regulatory light chain (RLC). These distributions show a complex relationship between the axial orientation of the RLC relative to the fiber axis and the azimuthal orientation of the RLC about its own axis. Maximum entropy analysis reveals limitations in available experimental data and supports the design of further probe angles to resolve details of the orientational distribution.

Conformational changes of contractile proteins and their role in muscle contraction

The review summarizes the results of studies on conformational changes in contractile proteins that occur during muscle contraction. Polarized fluorescence of tryptophan residues in actin and of fluorescent probes bound specifically to different sites on actin, myosin, or tropomyosin in muscle fibers was measured. The results show that the transition of actomyosin complex from the weak to the strong-binding state is accompanied by a change in the orientation of F-actin subunits with the C and N termini moving opposite to a large part of the subunit. Myosin light chains and some areas in the 20-kDa domain of myosin head move in the same direction as the C- and N-terminal regions of actin. It is established that troponin, caldesmon, calponin, and myosin systems of regulation of muscle contraction modify intramolecular actomyosin rearrangements in a Ca(2+)-dependent manner. The role of intramolecular movements of contractile proteins in muscle contraction is discussed.

Probes bound to myosin Cys-707 rotate during length transients in contraction

It is widely conjectured that muscle shortens because portions of myosin molecules (the "cross-bridges") impel the actin filament to which they transiently attach and that the impulses result from rotation of the cross-bridges. Crystallography indicates that a cross-bridge is articulated-consisting of a globular catalytic/actin-binding domain and a long lever arm that may rotate. Conveniently, a rhodamine probe with detectable attitude can be attached between the globular domain and the lever arm, enabling the observer to tell whether the anchoring region rotates. Well-established signature effects observed in shortening are tension changes resulting from the sudden release or quick stretch of active muscle fibers. In this investigation we found that closely correlated with such tension changes are changes in the attitude of the rhodamine probes. This correlation strongly supports the conjecture about how shortening is achieved.

Light chain-dependent myosin structural dynamics in solution investigated by transient electrical birefringence

The technique of transient electrical birefringence was used to compare some of the electric and structural dynamic properties of myosin subfragment 1 (S1(elc, rlc)), which has both the essential and regulatory light chains bound, to S1(elc), which has only an essential light chain. The rates of rotational Brownian motion indicate that S1(elc, rlc) is larger, as expected. The permanent electric dipole moment of S1(elc, rlc) is also larger, indicating that the regulatory light chain portion of S1(elc, rlc) has a dipole moment and that it is aligned head-to-tail with the dipole moment of the S1(elc) portion. The permanent electric dipoles decrease with increasing ionic strength, apparently because of ion binding to surface charges. Both S1(elc, rlc) and S1(elc) have intrinsic segmental flexibility, as detected by the ability to selectively align segments with a brief weak electric field. However, unlike S1(elc), which can be structurally distorted by the action of a brief strong electric field, S1(elc, rlc) is stiffer and cannot be distorted by fields as high as 7800 V/cm applied to its approximately 8000 D permanent electric dipole moment. The S1 . MgADP . Pi analog S1 . MgADP . Vi is smaller than S1 . MgADP, for both S1(elc, rlc) and S1(elc). Interestingly, the smaller, stiffer S1(elc, rlc) . MgADP . Vi complex retains intrinsic segmental flexibility. These results are discussed within a framework of current hypotheses of force-producing mechanisms that involve S1 segmental motion and/or the loss of cross-bridge flexibility during force production.

The conformation of xanthene dyes in the myosin sulfhydryl one binding site. Part I. Methods and model systems

Derivatives of the fluorescent probes fluorescein and rhodamine specifically and covalently modify the highly reactive thiol (SH1) of myosin subfragment 1 (S1). Both probes develop circular dichroism (CD) upon modification of SH1 at the visible absorption band of the chromophore. A model system of chiral complexing agents (aromatic chiral amines) interacting with fluorescein in solvent develops a CD signal that mimics that produced by S1. The model system suggests that a specific interaction of the probe with an aromatic chiral residue in the SH1 binding pocket induces the CD signal. Several other spectroscopic signals, including absorption and fluorescence intensity and anisotropy, characterize the fluorescein or rhodamine binding to SH1. A coupled dipole method is adapted to interpret these spectroscopic signals in terms of the probe-S1 complex conformation. The computation of the orientation of the principal hydrodynamic frame (PHF) of S1 from its crystallographic alpha-carbon backbone structure permits the known orientation of the probe in the PHF of S1 to further constrain the conformation of the probe-S1 complex. The coupled dipole interpretation of spectroscopic data combined with constraints relating the probe dipole orientation to the PHF of S1 determines the conformation of the probe-S1 complex. The methods developed here are applied to the spectroscopic signals from fluorescein or rhodamine in the SH1 binding site of S1 to obtain an atomic resolution model of the probe-S1 conformation [Ajtai and Burghardt, Biochemistry, 34 (1995) 15943-15952].

Fluorescence emission and anisotropy from rhodamine dimers

The absorption, fluorescence emission, and excitation fluorescence anisotropy spectra of a rhodamine dye in a water-glycerol solution at high concentration were investigated to determine spectroscopic properties of the ground state dimer. The combination of data from these spectra measured at several dye concentrations contained sufficient constraints on the model for dimer association to permit an estimate of: the association constant, the extinction coefficients, the relative fluorescence quantum yield, and the emission spectra of the monomeric and dimeric species. The rhodamine dimer is an efficient fluorescence emitter with fluorescence anisotropy equivalent to that of the pure monomeric species over the range of excitation wavelengths covering its two lowest energy transitions.