4.4 Article

Direct evidence supporting the existence of a helical dislocation in protofilament packing in the intermediate filaments of oxidized trichocyte keratin

Journal

JOURNAL OF STRUCTURAL BIOLOGY
Volume 204, Issue 3, Pages 491-497

Publisher

ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.jsb.2018.09.007

Keywords

Trichocyte keratin; Patterson function; Helical dislocation; IF-KAP binding; X-ray fiber diffraction; Meridional pattern; Axial electron density distribution

Funding

  1. Massey University

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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 angstrom. 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 z(a) similar to 74 angstrom and z(b) similar to 198 angstrom. 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.

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