The bZIP1 Transcription Factor Regulates Lipid Remodeling and Contributes to ER Stress Management in Chlamydomonas reinhardtii

Abstract

Youngsook Lee, ylee@postech.ac.kr. 24 25The authors declare there is no conflict of interests. 26 27Running title: CrbZIP1 modulates membrane lipids under ER stress 28 29One-Sentence Summary: The mRNA of a Chlamydomonas bZIP transcription factor is 30 spliced by CrIRE1 under ER stress, and the resulting protein protects Chlamydomonas cells 31 from ER stress by modulating lipid remodeling. 32 33 34Keywords: bZIP; Chlamydomonas; DGTS; ER stress; pinolenic acid 35 36Word count: 7958 words 37 38 39 2 ABSTRACT 40Endoplasmic reticulum (ER) stress is caused by the stress-induced accumulation of unfolded 41 proteins in the ER. Here, we identified proteins and lipids that function downstream of the 42 ER stress sensor inositol-requiring enzyme1 (CrIRE1), which contributes to ER stress 43 tolerance in Chlamydomonas reinhardtii. Treatment with the ER stress inducer tunicamycin 44 resulted in the splicing of a 32-nucleotide fragment of a bZIP transcription factor (CrbZIP1) 45 mRNA by CrIRE1, which resulted in the loss of the transmembrane domain in CrbZIP1, and 46 the translocation of CrbZIP1 from the ER to the nucleus. Mutants deficient in CrbZIP1 failed 47to induce the expression of the unfolded protein response genes and grew poorly under ER 48 stress. Levels of diacylglyceryltrimethylhomo-Ser (DGTS) and pinolenic acid (18:3Δ5,9,12) 49 increased in the parental strains but decreased in the crbzip1 mutants under ER stress. A yeast 50one-hybrid assay revealed that CrbZIP1 activated the expression of enzymes catalyzing the 51 biosynthesis of DGTS and pinolenic acid. Moreover, two independent alleles of crdes mutant, 52 which failed to synthesize pinolenic acid, were more sensitive to ER stress than were their 53 parental lines. Together, these results indicate that CrbZIP1 is a critical component of the ER 54 stress response mediated by CrIRE1 in Chlamydomonas that acts via lipid remodeling. 55 57Chlamydomonas reinhardtii is a model microalga widely used for studies of photosynthesis, 58 flagella biogenesis, and lipid metabolism (Harris, 2001). Similar to many other microalgae, 59Chlamydomonas produces a large amount of triacylglycerol (TAG) under stress conditions 60 (Merchant et al., 2012), and this prompted intensive studies of the mechanisms underlying 61 stress responses in this organism. The endoplasmic reticulum (ER) stress response is an 62 important stress response that is common to many eukaryotes. ER stress can be induced by 63 biotic and abiotic factors, such as pathogen infection, temperature, high salinity, wounding, 64 and ER stress inducing agents (Howell, 2013), and results in the accumulation of unfolded 65 proteins in the ER. In mammals, the unfolded proteins accumulated in the ER are sensed by 66 proteins in the ER membrane that transmit this information to the nucleus, inducing the 67 expression of genes and restoring normal protein metabolism. This process is called the 68 unfolded protein response (UPR). UPR genes encode molecular chaperones, folding enzymes, 69 lipid biosynthesis enzymes, and componen...

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