Comparison and contrast of plant, yeast, and mammalian ER stress and UPR

Abstract

The endoplasmic reticulum (ER) is a well-characterized protein folding mechanism in eukaryotic organisms. Many secretory and membrane proteins are folded in the ER before they are translocated to their functional destination. Various conditions, such as biotic, abiotic, or physiological stresses, lead to the accumulation of unfolded and misfolded proteins in the ER, resulting in ER stress. In response to ER stress, cells initiate a protective response called the unfolded protein response (UPR) to maintain cellular homeostasis. Previous studies suggest that inositol-requiring kinase 1 (IRE1) is a universal ER stress sensor in yeast, mammals, and plants. IRE1-mediated splicing of UPR transducers, such as HAC1, XBP1, and bZIP60, triggers the UPR in yeast, mammals, and plants, respectively. In mammals, activated transcription factor 6 and double stranded RNA-activated protein kinase-like ER kinases are involved in the UPR. In plants, the additional UPR transducers bZIP28 and bZIP17 are activated by Golgi-localized S1P and S2P proteases. Subsequently, these UPR transducers are exported to the nucleus and upregulate the expression of UPR-responsive genes encoding BiP, calreticulin, calnexin, protein disulfide isomerase, and glucose-regulated protein 94 to decrease the amount of misfolded proteins and induce endoplasmic reticulum-associated degradation. In plants, the UPR signaling pathway plays an important role in ER homeostasis and normal biological processes; however, the molecular mechanisms of the UPR in plants remain poorly understood. This paper provides an overview of the regulatory and signaling mechanisms of the UPR across kingdoms. In addition, the emerging role of the UPR in plant physiology and defense response will be discussed.