title: Visualization of a 21-nm axial periodicity in shadowed keratin filaments and neurofilaments.
abstract: Unidirectional and rotary shadowing techniques have been applied in studying the surface structure of two types of intermediate filaments. Keratin filaments and neurofilaments demonstrate a approximately 21-nm axial periodicity which probably indicates the helical pitch of the outer shell of the filament. Analysis of unidirectionally shadowed keratin showed that the helix is left-handed. The observation of a left-handed helix of 21-nm pitch supports the three-stranded protofilament model of Fraser, Macrae, and Suzuki (1976, J. Mol. Biol. 108:435-452), and indicates that keratin filaments probably consist of 10 three-stranded protofilaments surrounding a core of three such protofilaments, as predicted by models based on x-ray diffraction of hard keratin filaments. Neurofilaments do not demonstrate an easily identifiable hand, so their consistency with the model is, as yet, uncertain.
mesh_terms: D000818:Animals; D002417:Cattle; D003599:Cytoskeleton / Q000648:ultrastructure*; D007633:Keratins; D008854:Microscopy, Electron
chemical_list: D007633:Keratins