Managing the protein folding demands in the endoplasmic reticulum of plants

Abstract

Summary Endoplasmic reticulum (ER) stress occurs in plants during certain developmental stages or under adverse environmental conditions, as a result of the accumulation of unfolded or misfolded proteins in the ER. To minimize the accumulation of misfolded proteins in the ER, a protein quality control (PQC) system monitors protein folding and eliminates misfolded proteins through either ER‐associated protein degradation (ERAD) or autophagy. ER stress elicits the unfolded protein response (UPR), which enhances the operation in plant cells of the ER protein folding machinery and the PQC system. The UPR also reduces protein folding demands in the ER by degrading mRNAs encoding secretory proteins. In plants subjected to severe or chronic stress, UPR promotes programmed cell death (PCD). Progress in the field in recent years has provided insights into the regulatory networks and signaling mechanisms of the ER stress responses in plants. In addition, novel physiological functions of the ER stress responses in plants for coordinating plant growth and development with changing environment have been recently revealed.

ER-Anchored Transcription Factors bZIP17 and bZIP28 Regulate Root Elongation

“…Nonetheless, the UPR modulator roles of bZIP17 have been overlooked following 331 studies revealing that large parts of the canonical UPR regulon are regulated by bZIP28 332 and bZIP60 but not by bZIP17 (Liu and Howell, 2016;Angelos et al, 2017). In the 333 present study, we characterize a double mutant of bZIP17 and bZIP28, which provides 334 evidence that bZIP17 plays functionally redundant roles with bZIP28 based on the 335 drastic developmental disorders that were observed in the double mutant but not in the 336 single mutant (Fig.…”

“…These chaperones assist in the proper 91 folding and modification of nascent polypeptides to form functional proteins or reduce 92 the aggregated malformed protein level through a specific proteasome pathway, 93 ER-associated protein degradation (ERAD). Their stress-responsive expression is 94 largely dependent on bZIP28 and bZIP60, with these two bZIPs currently described as 95 master regulators of the UPR in plants (Liu and Howell, 2016;Angelos et al, 2017). 96…”

“…Previous studies have reported that the IRE1-bZIP60 arm regulates the 98 expression of SECRETORY 31A (SEC31A) (Deng et al, 2016) and the 99 SECRETION-ASSOCIATED RAS 1A (SAR1A)/SEC23A pair (Zeng et al, 2015), the 100 components of which contribute to male fertility and bZIP28 activation, respectively. In 101 addition to conserved UPR regulation in eukaryotes, several NAC-type transcription 102 factors have been reported to be bZIP targets, and are likely associated with a 103 plant-specific UPR (Liu and Howell, 2016). 104…”

Arabidopsis protein disulfide isomerase-8 is a type I endoplasmic reticulum transmembrane protein with thiol-disulfide oxidase activity

“…Misfolded proteins can impair cellular processes in a variety of ways, leading to the unfolded protein response (UPR) and ER stress [19, 30]. Due to the important role PDIs serve in catalyzing protein folding, the abnormal accumulation of misfolded proteins within the ER is accompanied by an increase in PDI expression and activity [12].…”

Dynamics of Autophagosome Formation

“…Autophagy is induced by ER stress, in which accumulation of unfolded and misfolded proteins within the ER activates the unfolded protein response (UPR; Liu and Howell, 2016). Although repression of TOR activity leads to activation of autophagy during some abiotic stresses, autophagy induced by ER stress seems to be independent of TOR (Pu et al, 2017), as autophagosomes are formed normally during ER stress in TOR overexpression lines.…”