| 1. |
- Urzhumtsev, Alexandre, et al.
(författare)
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Ultralow-resolution ab initio phasing of filamentous proteins : crystals from a six-Ig fragment of titin as a case study.
- 2008
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Ingår i: Acta Crystallographica Section D. - : Wiley-Blackwell Publishing Inc.. - 0907-4449 .- 1399-0047. ; 64:Pt 5, s. 478-486
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Tidskriftsartikel (refereegranskat)abstract
- Low-resolution diffraction data (resolution below 12 angstroms) from crystals of a filamentous six-Ig fragment of titin, I65-I70, were used in ab initio phasing with the aim of calculating its lattice packing and molecular envelope. Filamentous molecules, characterized by marked anisometry and idiosyncratic crystal lattices, have not been addressed before using this methodology. In this study, low-resolution phasing (19-122 angstroms) successfully identified the region of the unit cell occupied by the molecule. Phase extension to a higher resolution (12 angstroms) yielded regions of high density that corresponded either to the positions of individual Ig domains or to zones of dense intermolecular contacts, hindering the identification of individual domains and the interpretation of electron-density maps in terms of a molecular model. This problem resulted from the acutely uneven packing of the molecules in the crystal and it was further accentuated by the presence of partially disordered regions in the molecule. Addition of low-resolution reflections with phases computed ab initio to those obtained experimentally using MIRAS improved the initial electron-density maps of the atomic model, demonstrating the generic utility of low-resolution phases for the structure-elucidation process, even when individual molecules cannot be resolved in the lattice.
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| 2. |
- von Castelmur, Eleonore, et al.
(författare)
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A regular pattern of Ig super-motifs defines segmental flexibility as the elastic mechanism of the titin chain
- 2008
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - Washington, DC, United States : National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 105:4, s. 1186-1191
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Tidskriftsartikel (refereegranskat)abstract
- Myofibril elasticity, critical to muscle function, is dictated by the intrasarcomeric filament titin, which acts as a molecular spring. To date, the molecular events underlying the mechanics of the folded titin chain remain largely unknown. We have elucidated the crystal structure of the 6-Ig fragment I65-I70 from the elastic I-band fraction of titin and validated its conformation in solution using small angle x-ray scattering. The long-range properties of the chain have been visualized by electron microscopy on a 19-Ig fragment and modeled for the full skeletal tandem. Results show that conserved Ig-Ig transition motifs generate high-order in the structure of the filament, where conformationally stiff segments interspersed with pliant hinges form a regular pattern of dynamic super-motifs leading to segmental flexibility in the chain. Pliant hinges support molecular shape rearrangements that dominate chain behavior at moderate stretch, whereas stiffer segments predictably oppose high stretch forces upon full chain extension. There, librational entropy can be expected to act as an energy barrier to prevent Ig unfolding while, instead, triggering the unraveling of flanking springs formed by proline, glutamate, valine, and lysine (PEVK) sequences. We propose a mechanistic model based on freely jointed rigid segments that rationalizes the response to stretch of titin Ig-tandems according to molecular features.
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