The Cellular Response to Stress in the Endoplasmic Reticulum

Randal J. Kautman
The Department of Biological Chemistry and
the Howard Hughes Medical Institute
University of Michigan
Ann Arbor, MI 48109

In eukaryotic cells, accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER) activates signaling pathways to induce transcription in the nucleus and inhibit translation in the cytoplasm. We have studied the activation of two kinase cascades that provide fundamental roles in the cellular response to unfolded protein.

The interferon-inducible, double-stranded RNA-dependent serine/threonine protein kinase (PKR) plays a role in viral pathogenesis, cell growth, and differentiation and is implicated as a tumor suppressor gene. We have recently shown that stressful conditions that cause accumulation of protein in the ER elicit activation of PKR. In addition, activation of PKR in turn inhibits protein synthesis initiation through phosphorylation of the translation initiation factor2. Activation of PKR is necessary end sufficient to induce apoptosis in response to a variety of different stimuli including viral infection, serum depletion, TNF-a treatment, and inhibition of N-linked glycosylation by tunicamycin. Further studies demonstrated that the apoptotic response mediated through PKR is directed through eIF2 phosphorylation.

Accumulation of unfolded proteins in the ER results in transcriptional induction of a number of genes encoding ER protein chaperones. In Saccharomyces cerevisiae the transmembrane serine/threonine protein kinase receptor Irelp, is implicated as the sensor of unfolded proteins in the ER that transmits the signal from the ER to transcriptional activation in the nucleus. Activation of this kinase leads to a new spliced form of the rnRNA encoding a transcription factorHaclp that is responsible for transcriptional activation of genes encoding ER protein chaperones. Whereas unspliced Hac lp mRNA encodes a protein that is rapidly degraded, spliced Haclp rnRNA encodes a protein that is very stable. Using the yeast-interaction trap system we have identified a yeast transcriptional coactivator complex, Gcn5p/Ada, that interacts with Irelp and Haclp. GcnSp was recently shown to have histone acetylase activity. Deletion of GCN5 reduces the transcriptional induction in response to misfolded protein in the ER. These results are incorporated into a model where Irelp activation induces splicing of Haclp mRNA as well as engages GcnSp/Adato target Gcn5p/Ada/Haclp complex to the chromosomal loci that encode genes that are transcriptionally activated in response to unfolded protein in the ER. The targeting of Gcn5p to these loci results in local histone acetylation and opening of the chromatin to facilitate transcription.