The Molecular Architecture for the Intermediate Filaments of Hard α-Keratin Based on the Superlattice Data Obtained from a Study of Mammals Using Synchrotron Fibre Diffraction
Figure 1
(a) This figure shows the winding together of two hetero keratin fibrils to form a dimer. (b) This figure shows the winding together of two dimers in a Crick “knob in hole” packing. (c) This figure depicts the lattices that superimpose on the diffraction pattern resulting in the superlattice that is the resultant of the superposition of the seven lattices discussed below. The 47 nm lattice is associated with the distance between the beginning of the helical section of one tetramer and the beginning of the helical section of the next but one tetramer, for example, AB, BD, thus creating an obvious infinite and continuous lattice in the direction of the hair. The finite lattices (19.8 nm, 272 nm, and 12.4 nm), recorded by Wilk et al. [6], James et al. [7], Feughelman et al. [12], and James [11] are subsets of the projections of the 47 nm lattice being the projection of the 200 nm section of the helical section [15] plus and minus the nonhelical section [8, 11, 12]. All reflections from the two other lattices 7.8 nm and 15.6 nm representing the C and N terminal noncoiled ends and their sum [15] are buried under reflections of the other lattices. The repeat distance of 62.6 nm is the projection on the direction of the fibre of the complete tetramers, that is, the projection of the sum of the lengths of the helical and nonhelical sections of the tetramer, for example, AC, as it winds through 120°. Considering a line entering at A and leaving at C, the presence of an infinite lattice with spacing 62.6 nm is not so obvious. As the geometrical analysis shows [10] it is mathematically possible for one and only one condition, namely, that the distance between the IFs is three-times the radius of the IF. This is illustrated in Figure 2 where A, B, C, D, and E are successive points along the lattice. They are separated by ~63.1 nm which projects onto the direction of the hair as 62.6 nm. The insert is a view vertically downwards showing a sequence of lattice points of this infinite lattice traversing through the hexagonal array as it progress along the hair. (d) This figure shows the progress of the infinite 62.6 nm lattice as it progresses the length of the hexagonal array of intermediate filaments (IFs) in the sample. The unique hexagonal geometrical arrangement of the IFs, namely, that the centre to centre spacing of adjacent IFs is three-times the radius of the IF, and the fact that there is 60° between the linkage points on adjacent tetramers, give rise to an infinite repeating lattice, one set of points being A, B, C, D, E, and so forth, that has a projected spacing of 62.6 nm in the direction of the hair. The vertical view shows the sideways progression across the hexagonal array of successive points on the lattice as it moves along the length of the sample. Each point is separated along the length of the hair by 62.6 nm from its neighbours.
