Direct evidence supporting the existence of a helical dislocation in protofilament packing in the intermediate filaments of oxidized trichocyte keratin.
The X-ray diffraction patterns of quill and hair, as well as other trichocyte keratin appendages, contain meridional reflections that can be indexed on an axial repeat of 470 Å. Unusually, however, many of the expected orders are not observed. A possible explanation, proposed by Fraser and MacRae (1983), was that the intermediate filaments (IF) that constitute the fibrillar component of the filament/matrix texture consist of 4-chain protofilaments arranged on a surface lattice subject to a helical dislocation. The radial projection of the resulting 8-protofilament ribbon was defined in terms of a two-dimensional unit cell characterized by vectors (a, b) with axial projections za ∼ 74 Å and zb ∼ 198 Å. This situation resembles that found in microtubules, where helical dislocations in subunit packing are also encountered, leading to a so-called "seam" along their length (Metoz and Wade, 1997). In keratin, however, the protofilaments are helical so the seam is inclined to the axis of the IF. Here we report details of the Patterson function that provides independent evidence for both the helical dislocation and the dimensions of the surface lattice. In addition, the observed meridional X-ray amplitudes have been compared with those predicted by various models of the axial distribution of electron density. A new model, adapted from one previously proposed, fits the data significantly better than has heretofore proved possible. An interpretation of the model in terms of either specific keratin-associated-protein (KAP) binding or the retention of IF symmetry by a portion of the head and/or tail domains is suggested.
['Fraser|R D Bruce|RDB|', 'Parry|David A D|DAD|']
D000818:Animals D018360:Crystallography, X-Ray D003598:Cytoskeletal Proteins D006197:Hair D007382:Intermediate Filaments D053564:Keratins, Hair-Specific D008956:Models, Chemical D010084:Oxidation-Reduction D046328:Porcupines D014961:X-Ray Diffraction
Helical dislocation Trichocyte keratin Patterson function Axial electron density distribution IF-KAP binding Meridional pattern X-ray fiber diffraction