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. Expressed in skin during two hair growth cycles. Expression restricted to the cortical cells of hair follicles, appearing first in the cortical cells processing the flat nuclei located a few cells above the dermal papilla. Belongs to the KRTAP type 19 family.