The ability to maintain ER function and adapt to the demand for increased protein folding is a crucial part of the way in which cells respond to stress or damage. IRE1 and PERK are type I transmembrane protein kinases localized to the endoplasmic reticulum (ER) that are activated by the accumulation of unfolded proteins in the ER to signal transcriptional induction of genes encoding ER chaperones, such as BIP, which increases the total folding capacity. This pathway is called the unfolded protein response (UPR). IRE1 is essential to signal the UPR in S. cerevisiae, however, is dispensable for the mammalian UPR. Although absent in S. cerevisiae, in metazoan species PERK phosphorylates the a subunit of eukaryotic translation initiation factor 2 (eIF2a) to attenuate general protein synthesis, thereby reducing the amount of proteins that require folding. However, in mammalian cells PERK is also required to activate transcription of the UPR-responsive genes through preferential translation of selective transcription factors. To develop a genetic method for dissecting the UPR in higher eukaryotes, we are studying C. elegans. C. elegans has two BIP genes that are induced 2- to 10- fold upon ER stress, suggesting that the UPR exist in C. elegans. We used RT-PCR and RACE to clone
ire-1, the homolog of the IRE1 gene, and
pek-1, the homolog of the PERK gene. RNA interference (RNAi) showed that
ire-1 and
pek-1 are redundant, and mediate a process that is essential for worm development. The
ire-1(RNAi);
pek-1(RNAi) double mutants arrest as L2 larvae, and their intestines rapidly degrade as the animals age. When we studied the expression of
pek-1, with a promotor::GFP fusion construct, we found that it was strongly expressed in the intestine, as well as in several other tissues. Surprisingly, the UPR monitored by BIP induction remained intact in these double mutants, suggesting the existence of additional pathways that activate the UPR in C. elegans. However, at this point we cannot rule out that residual signaling through
ire-1 and/or
pek-1 occurs due to incomplete RNAi silencing. To address this question, we recently isolated a deletion mutant of
ire-1, and are now using it to identify new genes that act in this response.