Cryo-Electron Microscopy of Trichocyte (Hard α-Keratin) Intermediate Filaments Reveals a Low-Density Core

Abstract

Trichocyte intermediate filaments (IF) are the principal components of epidermal appendages such as hair and nail. Based on studies by a variety of techniques, it has been inferred that trichocyte IF are structurally similar to other kinds of IF. However, some basic structural attributes have yet to be established: in particular, it has remained unclear whether IF are hollow. We have examined trichocyte IF isolated from rat vibrissae and human hair follicles by electron microscopy. Scanning transmission electron microscopy of freeze-dried specimens yielded mass-per-unit-length values of ∼32 kDa/nm, with the human preparations also containing filaments at half this density, corresponding to two rather than four protofibrils. Radial density profiles calculated from cryo-electron micrographs of vitrified specimens preserved in a near-native state revealed a low-density region of ∼3 nm diameter around the filament axis. A minor species of filament with the same internal structure was surface-decorated with material arranged with a helical pitch length of 9.3 nm. These filaments appear to represent IF coated with associated proteins—perhaps, “high-sulfur” proteins—readied for incorporation into the filament-matrix biocomposite of the mature hair.