In the hair cortex, hair keratin intermediate filaments are embedded in an interfilamentous matrix, consisting of hair keratin-associated protein (KRTAP), which are essential for the formation of a rigid and resistant hair shaft through their extensive disulfide bond cross-linking with abundant cysteine residues of hair keratins. The matrix proteins include the high-sulfur and high-glycine-tyrosine keratins. Interacts with hair keratins. Expressed during the active phases of the hair cycle in the medulla and the inner root sheath of the forming hair. Also expressed in the upper layers of the epidermis of skin. Belongs to the KRTAP type 5 family.