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. Specific expression in the middle/upper hair cuticle. Has a low cysteine content when compared to the majority of known KAP family members. Possesses several C-terminal tyrosine-containing tandem decameric repeat structures. Belongs to the PMG family. It is uncertain whether Met-1 or Met-6 is the initiator.