Why would induction of the ER stress response play a protective role against PFTs? It is possible that PFTs somehow lead to the accumulation of unfolded proteins in a cell. For example, PFTs are known to perturb calcium homeostasis and changes in calcium homeostasis are known to affect protein folding [32, 33] . In this model, cells would respond to the toxin via p38 MAPK and turn on the UPR to anticipate and ameliorate the detrimental effects of unfolded proteins. Arguing against this model, however, is our data showing that sensitivity of the three arms of the UPR to Cry5B is different than their sensitivity to a global unfolder of ER proteins, tunicamycin. A second model is that activation of the ER stress response by Cry5B in a p38 MAPK dependent manner may prepare the cell to handle an altered biosynthetic load in the ER to defend against a toxin. For example, transcriptional array analysis indicate that over 1000 genes are differentially regulated in C. elegans by Cry5B ingestion [6] , which could in turn lead to significant changes in the protein load of the ER. A third model is 4(bn2) and glp-4(bn2);sek-1(km4) after 3 hours of exposure to either control (DMSO) or tunicamycin (2 mg/mL). This is a representative experiment of three independent experiments. (D) In vivo induction of hsp-4::GFP by Cry5B requires pmk-1 (p38 MAPK). The strains hsp-4::GFP and hsp-4::GFP;pmk-1(km25) were fed either control E. coli or E. coli expressing Cry5B for 8 hours and the expression of GFP was then analyzed. Cry5B induces GFP within the intestinal cells of the strain hsp-4::GFP but not in the strain containing the pmk-1(km25) mutant. The experiment was performed three times and representative worms are shown. Scale bar is 0.2 mm. (E) In vivo induction of hsp-4::GFP by tunicamycin does not require pmk-1 (p38 MAPK). The strains hsp-4::GFP and hsp-4::GFP;pmk-1(km25) were exposed to either control (DMSO) or tunicamycin (2 mg/mL) for 8 hours and the expression of GFP was then analyzed. Tunicamycin induces GFP throughout both the strains hsp-4::GFP and hsp-4::GFP;pmk-1(km25), including within the intestinal cells. The experiment was performed three times and representative worms are shown. Scale bar is 0.2 mm. (F) Downstream targets of the UPR require the p38 MAPK pathway for induction by PFT but not unfolded proteins. The fold change in the levels of hsp-4 and Y41C4A.11 mRNA transcripts by Cry5B and tunicamycin were determined for glp-4(bn2), glp-4(bn2);xbp-1(zc12) and glp-4(bn2);sek-1(km4) using real-time PCR. In addition, the fold change in ttm-2 transcripts was determined in response to Cry5B. Data are mean and standard deviation of three independent experiments. (G) Animals lacking sek-1 MAPKK are more sensitive to Cry5B than animals lacking xbp-1. Wild-type N2, sek-1(km4), and xbp-1(zc12) animals were placed on plates spread with E. coli transformed with empty vector (0%) or spread with empty vector E. coli diluted 9:1 (10%) or 3:1 (25%) with Cry5B-expressing E. coli (% thus gives toxin dose on a plate relative to undiluted Cry5B-expressing E. coli). The assay was initiated with L4 stage worms and photographs were taken 48 hours later. In the absence of Cry5B, the worms developed into dark, gravid, active, healthy adults. On 10% Cry5B-expressing E. coli, xbp-1(zc12) were slightly smaller than N2 but healthier than sek-1(km4), which were as small, pale, inactive, and severely intoxicated. On 25% Cry5B-expressing E. coli, xbp-1(zc12) was more intoxicated than N2 but not as intoxicated as sek-1(km4) animals. Scale bar is 0.2 mm. doi:10.1371/journal.ppat.1000176.g005