Ricin inhibits activation of the unfolded protein response by preventing splicing of the HAC1 mRNA.
Ricin A chain (RTA) inhibits protein synthesis by removing a specific adenine from the highly conserved alpha-sarcin/ricin loop in the large rRNA. Expression of RTA with its own signal sequence in yeast resulted in its translocation into the endoplasmic reticulum (ER) and subsequent glycosylation. Because RTA must unfold within the ER, it may be vulnerable to host defenses, such as the unfolded protein response (UPR). UPR was induced in cells expressing an active site mutant but not the wild type RTA, indicating that the active site of RTA played a role in perturbing the ER stress response. The inactive RTA without the signal sequence did not induce UPR, indicating that translocation into the ER was critical for induction of UPR. The wild type RTA inhibited activation of UPR not only due to ER stress induced by the protein itself but also by global effectors such as tunicamycin and dithiothreitol. Mature RTA without the signal sequence also inhibited UPR, providing evidence that inhibition of UPR occurred on the cytosolic face of the ER. RTA could not inhibit UPR when the spliced form of HAC1 mRNA was provided in trans, indicating that it had a direct effect on UPR upstream of HAC1-dependent transcriptional activation. Only the precursor form of HAC1 mRNA was detected in cells expressing RTA after exposure to ER stress, demonstrating that ricin inhibits activation of UPR by preventing HAC1 mRNA splicing. The RTA mutants that depurinated ribosomes but did not kill cells were not able to inhibit activation of UPR by tunicamycin, providing evidence that the inability to activate UPR in response to ER stress contributes to the cytotoxicity of ricin.
Parikh
Bijal A
BA
Biotechnology Center for Agriculture and the Environment and the Department of Plant Biology and Pathology, School of Environmental and Biological Sciences, Rutgers University, 59 Dudley Road, New Brunswick, NJ 08901-8520, USA.
Tortora
Andrew
A
Li
Xiao-Ping
XP
Tumer
Nilgun E
NE
D000900:Anti-Bacterial Agents / Q000494:pharmacology; D050976:Basic-Leucine Zipper Transcription Factors / Q000096:biosynthesis* / Q000235:genetics; D003593:Cytoplasm / Q000235:genetics / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D006031:Glycosylation; D009154:Mutation; D017510:Protein Folding; D021382:Protein Sorting Signals / Q000235:genetics; D021381:Protein Transport / Q000187:drug effects / Q000502:physiology; D012326:RNA Splicing / Q000187:drug effects / Q000235:genetics; D012331:RNA, Fungal / Q000096:biosynthesis* / Q000235:genetics; D012333:RNA, Messenger / Q000096:biosynthesis* / Q000235:genetics; D012335:RNA, Ribosomal / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000096:biosynthesis* / Q000235:genetics; D012276:Ricin / Q000096:biosynthesis* / Q000235:genetics; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000096:biosynthesis* / Q000235:genetics; D014158:Transcription, Genetic / Q000187:drug effects / Q000235:genetics; D014415:Tunicamycin / Q000494:pharmacology
D000900:Anti-Bacterial Agents; D050976:Basic-Leucine Zipper Transcription Factors; C091341:HAC1 protein, S cerevisiae; D021382:Protein Sorting Signals; D012331:RNA, Fungal; D012333:RNA, Messenger; D012335:RNA, Ribosomal; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D014415:Tunicamycin; D012276:Ricin