ER stress signaling in plants

Abstract

The endoplasmic reticulum (ER) is the entrance and the first way station in the protein secretory pathway in plant cells. Newly synthesized proteins enter the ER in an unfolded state and are folded by the protein-folding machinery in the ER. The protein-folding machinery in the ER monitors the folding status and modification state of oligosaccharides on glycoproteins as they advance through the folding process. Protein folding is very sensitive to environmental conditions, and adverse conditions can result in protein misfolding which produces ER stress. Misfolded proteins are sensed by an ER quality control (ERQC) system and eliminated by the ER-associated degradation (ERAD) system, which transports misfolded proteins out of the ER and sends them for degradation to the 26S proteasome. When the load of misfolded proteins increases under stress conditions, it sets off an alarm called the unfolded protein response (UPR). The UPR involves the upregulation of stress-response genes that boost the capacity of the folding machinery and the ERAD system. The UPR signaling pathway has two "arms" - one arm involving ER membrane-associated transcription factors and another that involves messenger RNA splicing by the splicing factor called IRE1. ER stress activates the membrane-associated transcription factor arm of the UPR signaling pathway by mobilizing the factors from the ER to the nucleus where they upregulate stress-response genes. ER stress activates IRE1 to splice a messenger RNA encoding a transcription factor that also upregulates stress-response genes. Mild ER stress elicits autophagy through a signaling pathway that involves IRE1. Severe ER stress can lead to cell death.