Cryo-electron microscopy of trichocyte (hard alpha-keratin) intermediate filaments reveals a low-density core.
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 approximately 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 approximately 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.
Watts
Norman R
NR
Laboratory of Structural Biology Research, National Institutes of Health, Bethesda, Maryland 20892-8025, USA.
Jones
Leslie N
LN
Cheng
Naiqian
N
Wall
Joseph S
JS
Parry
David A D
DA
Steven
Alasdair C
AC
D000818:Animals; D020285:Cryoelectron Microscopy / Q000379:methods*; D019281:Dimerization; D006197:Hair / Q000648:ultrastructure; D006801:Humans; D007382:Intermediate Filaments / Q000648:ultrastructure; D007633:Keratins / Q000235:genetics / Q000378:metabolism / Q000648:ultrastructure*; D008855:Microscopy, Electron, Scanning; D008954:Models, Biological; D011110:Polymorphism, Genetic; D017434:Protein Structure, Tertiary; D051381:Rats
D007633:Keratins