De novo filament formation by human hair keratins K85 and K35 follows a filament development pattern distinct from cytokeratin filament networks.

In human hair follicles, the hair-forming cells express 16 hair keratin genes depending on the differentiation stages. K85 and K35 are the first hair keratins expressed in cortical cells at the early stage of the differentiation. Two types of mutations in the gene encoding K85 are associated with ectodermal dysplasia of hair and nail type. Here, we transfected cultured SW-13 cells with human K85 and K35 genes and characterized filament formation. The K85-K35 pair formed short filaments in the cytoplasm, which gradually elongated and became thicker and entangled around the nucleus, indicating that K85-K35 promotes lateral association of short intermediate filaments (IFs) into bundles but cannot form IF networks in the cytoplasm. Of the K85 mutations related to ectodermal dysplasia of hair and nail type, a two-nucleotide (C T ) deletion (delCT) in the protein tail domain of K85 interfered with the K85-K35 filament formation and gave only aggregates, whereas a missense mutation (233A>G) that replaces Arg with His (R78H) in the head domain of K85 did not interfere with the filament formation. Transfection of cultured MCF-7 cells with all the hair keratin gene combinations, K85-K35, K85(R78H)-K35 and K85(delCT)-K35, as well as the individual hair keratin genes, formed well-developed cytoplasmic IF networks, probably by incorporating into the endogenous cytokeratin IF networks. Thus, the unique de novo assembly properties of the K85-K35 pair might play a key role in the early stage of hair formation.