The structure of human trichohyalin. Potential multiple roles as a functional EF-hand-like calcium-binding protein, a cornified cell envelope precursor, and an intermediate filament-associated (cross-linking) protein.

Trichohyalin is an 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 and is a known substrate of transglutaminases. We have determined the full-length sequence of human trichohyalin by use of RNA-mediated anchored polymerase chain reaction methods and from a genomic clone and analyzed its potential secondary structure. We show here that trichohyalin may have at least three important functions in these cells. The protein of 248 kDa is unusual in that it contains one of the highest contents of charged residues of any protein. Of several defined 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 between successive turns of the alpha-helix. Domain 6 is the most regular and may bind KIF directly by ionic interactions. Domains 5 and 7 are less well organized and may introduce folds in the molecule. Thus, human trichohyalin is predicted to be an elongated flexible rod at least 215 nm long and to function as a KIF-associated protein by cross-linking the filaments in loose networks. In addition, trichohyalin is similar to, but several times longer than, involucrin, a known cell envelope constituent, so that together, involucrin and trichohyalin may serve as scaffold proteins in the organization of the cell envelope of these cells or even anchor the cell envelope to the KIF network. Finally, trichohyalin possesses a pair of functional calcium-binding domains of the EF-hand type at its amino terminus that may be involved in its calcium-dependent postsynthetic processing during terminal differentiation.