Hard α-keratin intermediate filaments: an alternative interpretation of the low-angle equatorial X-ray diffraction pattern, and the axial disposition of putative disulphide bonds in the intra- and inter-protofilamentous networks

Abstract

STEM and electron microscopy have indicated that intermediate filaments (IF) may contain protofilaments and protofibrils, possibly with four and eight chains in section, respectively. Two important questions related to this observation but pertaining specifically to the structure of the hard α-keratin IF have been addressed in this work. The first relates to the apparently smaller number of chains in section (and hence protofilaments and/or protofibrils) in hard α-keratin IF as compared to epidermal keratin, vimentin and neurofilament IF. From the low-angle equatorial X-ray diffraction pattern the observed maxima were interpreted by others as arising from the Fourier transform of a uniformly dense cylinder of diameter 7.45 nm and, by implication, containing about 23 chains in section. Here it is shown that a simple, alternative density distribution for the IF is compatible with both the positions of the observed equatorial diffraction maxima and with 32 chains in section, thus allowing the possibility that IF from diverse sources (hard α-keratin, epidermal keratin, vimentin and neurofilaments) have a common number of chains in section and hence the same number of subfilamentous elements. The second point relates to the stabilisation of the protofilaments and/or protofibrils. It has been shown that the relative axial alignment of molecules in hard α-keratin IF is similar but not identical to that in both epidermal keratin and vimentin IF. A key difference involves the presence of a small gap (about 1.3-1.4 nm) between similarly directed molecules as distinct from the head-to-tail overlap between parallel molecules that appears to be a characteristic of all other IF, and which is believed to have a major role in stabilising these assemblies. In contrast, it was proposed that the hard α-keratin IF gains its stability in part through a network of intermolecular disulphide bonds. Here it is shown that disulphide bonds may exist between spatially-adjacent molecules one, two, three or four apart (and beyond) in the proposed surface lattice structure. This indicates the possibility that subfibrillar structural elements consisting of discrete groupings of disulphide-bonded molecules may be present in hard α-keratin IF