Essential for the proper assembly of type I and type II keratin protein complexes and formation of keratin intermediate filaments in the inner root sheath (irs) (By similarity). Plays a role in the cytoskeleton organization (By similarity). Heterodimer of a type I and a type II keratin. Heterodimer with type II keratin KRT5 leading to the formation of keratin intermediate filament (KIF) network. Interacts with KRT6A to form filaments. Expressed in skin and wool follicle. Expression localized to the inner root sheath of wool follicle. There are two types of cytoskeletal and microfibrillar keratin: I (acidic; 40-55 kDa) and II (neutral to basic; 56-70 kDa). Belongs to the intermediate filament family.