Mammalian keratin filaments are produced by the lateral and longitudinal aggregation of subunits, such as tetrameric protofilaments and octameric protofibrils (Aebi et al. 1983). The extent of aggregation depends on the pH and osmolarity of the surrounding cytoplasm (Yamada et al. 2002, Magin et al. 2007). Filaments have a cross-section of 32 keratin molecules (Jones et al. 1997).