Trichocyte Keratin-Associated Proteins (KAPs)

The trichocyte (hard α-) keratins are epidermal appendages (hair, wool, hoof, horn, claw, baleen and quill) with a classic filament-matrix composite structure. In human hair, for example, keratin intermediate filaments (IF) of diameter 7.5 nm are embedded in a matrix formed from at least 89 different types of keratin-associated proteins (KAPs). The latter fall into three families, generally defined in terms of their cysteine residue or glycine plus tyrosine residue content. The KAPs, which infiltrate the space between the IF, are recognized as having especially important roles in the organisation of the IF into macrofibrils, in determining some of the most important physical attributes of the fully-keratinised hair fibre, including its hardness, toughness and pliability, and in linking IF to one another, either directly or indirectly, with a resultant increase in durability and resistance to degradation by microorganisms. Sequence data for many KAPs are now available, and repeating motifs of varying extent have been observed in a number of them. Little, however, is known about their three-dimensional structures, though modelling has indicated that some local structural regularity is likely to exist. Current data suggest that the KAPs in vivo may adopt a variety of energetically-similar conformations stabilized predominantly by intramolecular disulfide bonds. The role of KAPs in hair diseases relates more to modulation in gene expression than to point mutations, in contrast to that observed for the IF proteins.