Chapter 1 31 Keratin-associated Proteins (KAPS) In the separation of solubilized hard keratin proteins by polyacrylamide gel electrophoresis (c.f. Figure 11) two major distinct groups of low-sulphur proteins, type-I and type-II, have been identified as having been derived mainly from the intermediate filaments of the keratin composite. We now know that 11 type-I and 6 type-II proteins are expressed in the hair follicle and incorporated into the human hair shaft.48 What was not type-I or type-II in the electrophoretic map has tended to be classified as keratin-associated protein or KAP. Of these latter proteins there are three major groups namely, high sulphur proteins (HS), ultra-high sulphur protein (UHS) and high glycine/tyrosine proteins (HGT). The distinction, in terms of sulphur content, between “high” and “ultra-high” has tended to be arbitrarily set at whether the cysteine content of the protein is below or above 30 moles%. A total of 85 KAP genes have been identified in the human genome, the majority of which are transcriptionally active. Of these, 68 fall into the class of determining either high or ultra-high hair KAPs and 17 as high glycine/tyrosine.49 A great deal is now known about the amino acid sequences of these proteins and the cellular compartments of the hair shaft containing them.49 On the other hand, in contrast to the KIF proteins, little is known about the molecular conformation of the KAPs and the precise roles they play in overall fiber structure. This said, there seems little doubt that many of the sulphur-rich and HGT KAPs form the matrix of the keratin composite of the cortex within which the KIFs are embedded. In this respect the cystine-rich KAPs probably engage in disulphide cross-linking at the surfaces of the KIFs with the cystine residues at the two ends of the KIF proteins (c.f. Figures 12 and 14). It is likely some of the UHS KAPs are contained within the exocuticle and A-layers of the fiber cuticle and, by dint of considerable intermolecular disulphide cross-linking, are responsible for the considerable hardness of these components.