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PubMed2000

A small-angle X-ray solution scattering study of bovine alpha-crystallin.

The native high molecular mass form of alpha-crystallin, the most important soluble protein in the eye lens, and its low molecular mass form obtained at 37 degrees C in dilute solutions were investigated by synchrotron radiation small-angle X-ray scattering. The alpha-crystallin solutions are polydisperse and good fits to the experimental data can be obtained using distributions of spheres with radii varying between about 5 and 10 nm. In spite of the polydispersity, two different ab initio methods were used to retrieve low resolution shapes from the scattering data. These shapes correspond to the z-average structure of the oligomers. In the absence of any symmetry constraints, the scattering curves of the two forms of alpha-crystallin yield bean-like shapes. The shape corresponding to the low molecular mass form has about 20% less mass at the periphery. Imposing tetrahedral symmetry on the average structures worsens the fit to the experimental data. We emphasized the apparent contradiction between hydrodynamic and molecular properties of alpha-crystallin. An explanation was put forward based on the presence of solvent-exposed flexible C-terminal extensions. We present two bead models ('hollow globule with tentacles' and 'bean with tentacles') based on NMR and cryo-electron microscopy studies and discuss how well they correspond with our data from X-ray scattering, light scattering and analytical ultracentrifugation.

Animals↗
PubMed2003

Dynamic and static properties of the concentrated micellar solution and gel phase of a triblock copolymer in water.

The dynamic and static behavior of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer "P94" is studied in aqueous solution. This polymer forms transparent micellar phase structures through the whole phase diagram. The characteristics of these structures can be tuned by variation of temperature and concentration. The transparent gel at high concentrations and at ambient temperatures is the main interest in this study. The transition from the ergodic solution to this nonergodic gel or glass state can be reached in two ways: at constant temperature with an increase of concentration or at constant concentration with increasing temperature. We characterize the system in both ways with dynamic and static light scattering and with small-angle x-ray scattering experiments. At 40 degrees C, no concentration dependent change in the micellar structure of the system can be detected from the small-angle x-ray scattering results. Even in the gel we find globular particles. This is important for the evaluation and interpretation of the dynamic light scattering measurements. In the ergodic phase region at constant temperature, we find that a ratio of 2.17 gives a perfect fit to the experimental data when we allow the effective volume fraction to be proportional to the mass fraction. In the gel region we find two diffusional decays in the intensity correlation functions and an arrested part. The diffusion coefficients from the faster decay stay constant inside the gel and the nonergodicity parameter increases linearly with concentration. Both, the fast and slow diffusional modes show no angular dependence of their scattering intensities, indicating their origin from small objects, while the arrested mode comes, as expected, from structures larger than the resolution of the experiments.

Journal Article↗
PubMed2005

Solution structure of the chicken skeletal muscle troponin complex via small-angle neutron and X-ray scattering.

Troponin is a Ca2+-sensitive switch that regulates the contraction of vertebrate striated muscle by participating in a series of conformational events within the actin-based thin filament. Troponin is a heterotrimeric complex consisting of a Ca2+-binding subunit (TnC), an inhibitory subunit (TnI), and a tropomyosin-binding subunit (TnT). Ternary troponin complexes have been produced by assembling recombinant chicken skeletal muscle TnC, TnI and the C-terminal portion of TnT known as TnT2. A full set of small-angle neutron scattering data has been collected from TnC-TnI-TnT2 ternary complexes, in which all possible combinations of the subunits have been deuterated, in both the +Ca2+ and -Ca2+ states. Small-angle X-ray scattering data were also collected from the same troponin TnC-TnI-TnT2 complex. Guinier analysis shows that the complex is monomeric in solution and that there is a large change in the radius of gyration of TnI when it goes from the +Ca2+ to the -Ca2+ state. Starting with a model based on the human cardiac troponin crystal structure, a rigid-body Monte Carlo optimization procedure was used to yield models of chicken skeletal muscle troponin, in solution, in the presence and in the absence of regulatory calcium. The optimization was carried out simultaneously against all of the scattering data sets. The optimized models show significant differences when compared to the cardiac troponin crystal structure in the +Ca2+ state and provide a structural model for the switch between +Ca2+ and -Ca2+ states. A key feature is that TnC adopts a dumbbell conformation in both the +Ca2+ and -Ca2+ states. More importantly, the data for the -Ca2+ state suggest a long extension of the troponin IT arm, consisting mainly of TnI. Thus, the troponin complex undergoes a large structural change triggered by Ca2+ binding.

Animals↗