Hair Follicle Differentiation: Expression, Structure, and Evolutionary Conservation of the Hair Type II Keratin Intermediate Filament Gene Family

The study characterized the type II keratin intermediate filament (IF) gene family, which was expressed early in hair follicle differentiation. In sheep wool, four type II IF proteins were identified, with one gene fully sequenced, encoding a 55 kDa protein named KU-9. These genes shared a similar exon/intron structure with epidermal type II keratin IF genes and were expressed in hair cortical cells, but not in the outer root sheath, inner root sheath, or medulla. The type II IF genes were sequentially activated during hair keratinocyte differentiation, starting near the dermal papilla and continuing as keratinocytes migrated upwards. A fourth gene was activated later. The genes with similar expression patterns were closely related in sequence, with conserved promoter elements, including a novel element also found in a coexpressed type I IF gene and three other hair keratin genes.