The Structure and Chemistry of Human Hair 26 A key question is, how are the coiled-coil dimers arranged within each intermediate filament? Not surprisingly offset alignment between pairs of the coiled-coil dimers provides a means for longitudinal assembly to filaments of semi-infinite length. Similarly such a tetramer would provide for radial growth of the filament but what is it that determines the overall diameter of the filament will be fixed at about 70 Å? By a remarkable feat of analytical protein chemistry, the late Peter Steinert at NIH Bethesda and collaborators gained information seemingly providing knowledge about the packing of the proteins in keratin intermediate filaments.62 Their method was to take the purified type-I and corresponding type-II keratin proteins and allow them to aggregate into intermediate filaments. The filaments were treated with a cleavable reagent, disulphosuccinimidyl tartrate, which served the purpose of cross-linking the ε-amino groups of lysyl residues only where these were juxtaposed. The cross-linked material was subjected to limited enzymatic proteolysis and cross- linked fragments were isolated and purified. For each of these latter fragments the cross-link was cleaved and the amino acid sequences in the peptides at either end of the cross-link were determined. Figure 13. View down the axis of the KIF type I/type II coiled-coil dimer. Figure 14. The KIF type I/type II coiled-coil dimer showing the arrangement of the various components along the length of the unit.