Intermediate filament-associated protein that associates in regular arrays with keratin intermediate filaments (KIF) of the inner root sheath cells of the hair follicle and the granular layer of the epidermis. It later becomes cross-linked to KIF by isodipeptide bonds. It may serve as scaffold protein, together with involucrin, in the organization of the cell envelope or even anchor the cell envelope to the KIF network. It may be involved in its own calcium-dependent postsynthetic processing during terminal differentiation. Homodimer. Additional isoforms seem to exist. Found in the hard keratinizing tissues such as the inner root sheath (IRS) of hair follicles and medulla, and in the epithelia of the tongue, hoof and rumen. Consists of nine domains. Domain 1 contains two EF-hand calcium-binding domains. Domains 2-4, 6, and 8 are almost entirely alpha-helical, configured as a series of peptide repeats of varying regularity, and are thought to form a single-stranded alpha-helical rod stabilized by ionic interactions. Domain 6 is the most regular and may bind KIF directly by ionic interactions. Domains 5 and 7 are less well organized and may induce folds in the molecule. Domain 9 contains the C-terminus, conserved among different species. Substrate of transglutaminase. Some 200 arginines are probably converted to citrullines by peptidylarginine deimidase. Belongs to the S100-fused protein family.