Function of Gcn4 during the unfolded protein response

Abstract

Endoplasmic reticulum (ER) stress, resulting from an accumulation of misfolded proteins, can induce a mechanism termed the unfolded protein response (UPR). In Saccharomyces cerevisiae, it is the inositol-requiring 1 (Ire1) pathway that is implicated in the UPR to restore cellular homeostasis. Upon activation, Ire1 splices HAC1 mRNA to produce HAC1i (induced), which is translated into the protein Hac1. This basic leucine zipper (bZIP) transcription factor binds to UPR-associated genes that contain an unfolded protein response element (UPRE), ultimately initiating transcription in response to ER stress. Hac1 exists as a bZIP transcription factor that is traditionally thought to bind to UPREs as a homodimer. Previous work has however suggested that Hac1 may exist as a heterodimer with another bZIP transcription factor, general control nonderepressible 4 (Gcn4), which is activated by general control nonderepressible 2 (Gcn2). Gcn4 is conventionally associated with the general amino acid control (GAAC) and is produced under an imbalance of amino acids. This study aimed to explore the requirement of Gcn4 in the UPR. Under a balanced provision of amino acids, GCN2 and GCN4 S. cerevisiae deletion strains did not show a decrease in expression of a KAR2-lacZ reporter from a Z691 plasmid. Under an imbalance of amino acids, both deletion strains exhibited reduced activity of this reporter. However, after correction for translational efficiency using a GCN4∆4uORF-lacZ reporter from a p227 plasmid, no difference was observed, suggesting Gcn2 and Gcn4 have no role in the UPR. This work could therefore have importance in supporting previous studies that suggest that Hac1 and Gcn4 have distinct roles in different regulatory pathways within S. cerevisiae.