Genes pulse with activity

Signaling from keratins B eing supportive is just not enough for a keratin. Seyun Kim, Pauline Wong, and Pierre Coulombe (Johns Hopkins University, Baltimore, MD) fi nd 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 fi laments. Coulombe’s group had found that embryos lacking K17 did not heal wounds properly. They now fi nd 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 mTORactivated protein synthesis. At least one known contributor to mTOR activation, the 14-3-3σ adaptor protein, was found with cytoplasmic K17 fi laments 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 celland context-specifi c regulation. Do we need 50 different keratins just for a mechanical function? Probably not.” Reference: Kim, S., et al. 2006. Nature. 441:362–365. Genes pulse with act iv i ty J onathan Chubb, Robert Singer, and colleagues (Albert Einstein College of Medicine, Bronx, NY) have devised a method to view transcription inside individual cells. By removing the effects of population averaging, the method reveals transcriptional pulses of a eukaryotic developmental gene. Transcription was visualized by inserting upstream stem loops into an endogenous developmentally regulated Dictyostelium gene called dscA. The nascent transcripts were then detected by a GFP fusion protein that binds to the RNA stem loops. Single-cell analyses revealed discontinuous transcription, or pulses, rather than smooth, uniform expression. Determining the pulse mechanism will require more investigation, but possibilities include reversible chromatin modifi cations or variations in transcription factor binding. Pulsing would provide exquisite regulatory control, suggests Singer. “A burst that makes a lot of message might overshoot the amount of protein needed. That can have deleterious effects.” Better to turn on for a fl icker, wait to equilibrate, then cycle back on if more protein is needed. dscA is induced as Dictyostelium form fruiting bodies. This increase did not come from stronger or more frequent pulses, but rather from more pulsing cells. The new recruits were often clustered, suggesting either that the cluster is seeing the same developmental stimulus or that expressing cells tell their neighbors to join in. Whether constitutive genes also pulse remains to be determined. Reference: Chubb, J.R., et al. 2006. Curr. Biol. 16:1018–1025. GFP reveals a spot of dscA transcription (light blue). C H U BB / EL SE VI ER