Being supportive is just not enough for a keratin. Seyun Kim, Pauline Wong, and Pierre Coulombe (Johns Hopkins University, Baltimore, MD) find that Keratin 17 (K17) also takes on signaling duties for cell growth during the wound response.
Keratins are a family of structural proteins that form intermediate filaments. Coulombe's group had found that embryos lacking K17 did not heal wounds properly. They now find that these problems do not stem from structural issues.
Wound-adjacent epithelial cells usually turn on K17, grow larger, reduce adhesion, and polarize before migrating into the wound site. This marked cell growth was noticeably stunted, however, in K17 mutants, and the authors traced this defect to a block in mTOR-activated protein synthesis.
At least one known contributor to mTOR activation, the 14-3-3σ adaptor protein, was found with cytoplasmic K17 filaments in wild-type keratinocytes. In cells lacking K17, however, 14-3-3σ was mostly nuclear. In the cytoplasm, 14-3-3σ might bring various components of the mTOR pathway to K17, though the exact mechanism of translational up-regulation by the keratin is unclear.
Coulombe has also found that keratins delay apoptosis in the hair follicle. Such signaling functions, he says, “might be the missing link towards accounting for why keratins exhibit such exquisite cell- and context-specific regulation. Do we need 50 different keratins just for a mechanical function? Probably not.”