In the hair cortex, hair keratin intermediate filaments are embedded in an interfilamentous matrix, consisting of hair keratin-associated proteins (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. Is essentially restricted to only one vertical half of the hair forming compartment and in beard hairs is absent from the central medulla. Belongs to the KRTAP type 8 family.