Keratin

Keratins (formerly also called cytokeratins ) are filament-forming proteins of epithelial cells and are essential for normal tissue structure and function. Keratin genes account for the majority of intermediate filament genes in the human genome and form the two largest sequence homology groups, type I and II, of this large multigene family. Their expression patterns are highly differentiation-specific, suggesting functional differences.

Keratins are differentiated on the basis of molecular configuration (alpha-helix or beta-sheet structures) into

• alpha-keratins
• and
• beta-keratins.

Alpha-keratins: Alpha-keratins are the main component of the stratum corneum of the epidermis and of hair and nails. The strength of alpha-keratins is enhanced by fiber formation. Here, the individual amino acid chains form a right-handed alpha helix. Several of these helices form a left-handed superhelix, a protofibril. Several of these protofibrils in turn unite to form microfibrils, which now organize themselves into keratin bundles and in this bundling are called macrofibrils. The consolidation of the collagen fibers takes place through cross-links by means of disulfide bridges. Thus, the keratin in horny material and nails is more cross-linked than the keratin in hair. In various diseases, e.g. in the disease group of ichthyoses and palmoplantar keratoses, different disorders of keratin structure and function occur as a result of mutations in the associated genes (KRT genes).

• Intracellularly, i.e. before the actual cornification process, alpha-keratins (= cytokeratins) are present in the form of loosely organized keratin filaments. Alpha-keratins belong to the intermediate filaments, which together with microtubules and microfilaments form the cytoskeleton of eukaryotic cells (see below Cytoskeleton). Currently, 20 different alpha-keratins are known, with molecular masses ranging from 40 to 68 kDa.
• KRT1-KRT8 are considered to belong to the neutral-basic type-A subfamily, KRT9-KRT20 represent the acidic type-B subfamily. Sequence homologies to M-proteins of streptococci are present in keratin 17. This structural similarity plays a role in the pathogenesis of psoriasis and other infection-triggered dermatoses.
• Alpha-keratins form in pairs in the intermediate filaments (see cytoskeleton below) a heterodimer complex consisting of a type-A and a type-B cytokeratin. The distribution pattern of these complexes differs considerably in individual epithelial cells, so that antibody detection against the keratin1-keratin20 subtypes can be used to narrow down the origin of these cells. This proof of origin can be used to define the genesis of tumor cells by antibody application.

Beta-keratins:

• Examples of beta-keratins found in nature are the silk protein of spider webs and silk. Unlike alpha- or cytokeratins, beta-keratins are not intracellular structural proteins but excretory products of the spider glands. Because of its folded sheet structure, beta-keratin is less extensible than the helically structured alpha-keratins.

1. Hesse M et al. (2001) Genes for intermediate filament proteins and the draft sequence of the human genome: novel keratin genes and a surprisingly high number of pseudogenes related to keratins 8 and 18. J Cell Sci 114:2569-2575.
2. Hesse MA et al (2004) Comprehensive analysis of keratin gene clusters in humans and rodents. Eur J Cell Biol 83:19-26.
3. Moll R. et al. (1982) The catalog of human cytokeratins: patterns of expression in normal epithelia, tumors and cultured cells. Cell. 31:11-14.
4. Schäkel K et al (2016) Pathogenesis of psoriasis. Dermatologist 67: 422-432