Identification of the C-terminal tail domain of AHF/trichohyalin as the critical site for modulation of the keratin filamentous meshwork in the keratinocyte.

Background: AHF/trichohyalin is a large structural protein abundant in the inner root sheath (IRS) of anagenic hair follicles, which has been thought to mediate the keratin filamentous assembly. However, its functional mechanism is largely unknown.

Objective: This study aimed at the identification of the key domain in AHF for keratin association and the establishment of a plausible mechanism for the modulation of the keratin meshwork.

Methods: Several keratinocyte cell lines were introduced with the full length or several mutants of AHF, together with IRS-specific keratin krt31, and the profile of the AHF granules and the cellular behaviors were carefully analyzed.

Results: Full length of AHF formed small round granules that clearly bound to and aligned on the exogenous keratin filaments in the keratinocytes, severely affected cellular growth, mobility and shape. Intriguingly, the removal of only 6 amino acids around the C-terminal tail of AHF resulted not only in the complete loss of its keratin adherent ability but also in a dramatic enlargement of the granules.

Conclusion: We propose a model for cytoskeletal modulation in the IRS of anagenic hair follicles: AHF latches onto the keratin bundles by its C-terminus and rearranges the keratin meshwork by intrinsic cohesive activity for the granule formation.