4 Cytokeratin 7 and 20 Agnieszka

Cytokeratins (CK or CKs) are the largest complex group of IF proteins. These are proteins, that are crucial in the development and differentiation of epithelial cells, as well as essential for normal tissue structure and function. The first keratin protein nomenclature was published by Moll et al. (1982) and it has been repeatedly updated in recent years (Schweizer et al., 2006). Based on the results of ongoing research in human and other vertebrates, Szeverenyi et al. (2008) published a comprehensive catalogue of the human keratins, their amino acid sequence, the nucleotide sequence of the keratin genes in humans, as well as the same data of the orthologue keratins and keratin genes in various vertebrate species. Based on their molecular weights and isoelectric points CKs were catalogued and divided into two groups: type I and type II cytokeratins. The type I cytokeratins consist of acidic, low molecular weight (40-56.5 kDa) proteins including cytokeratins K9-K28 and hair keratins K31-K40. The type II cytokeratins consist of basic, high molecular weight (52-67 kDa) proteins including cytokeratins K1-K8, cytokeratins K71-K80 and hair keratins K81-K86 (Bragulla & Homberger, 2009; Iwatsuki & Suda, 2010; Moll et al., 2008; Schweizer et al., 2006). Cytokeratins are relatively resistant to degradation and show highly specific phenotypic expression depending on the type and differentiation of the epithelial cells. Hence, the different types of epithelia have specific for them pattern of keratin expression.