A rice transmembrane bZIP transcription factor, OsbZIP39, regulates the endoplasmic reticulum stress response

The endoplasmic reticulum (ER) responds to the accumulation of unfolded proteins in its lumen (ER stress) by activating intracellular signal transduction pathways. These pathways are known as the ER stress response or the unfolded protein response. In this study, three rice basic leucine zipper (bZIP) transcription factors (OsbZIP39, OsbZIP50 and OsbZIP60) containing putative transmembrane domains (TMDs) in their C-terminal regions were identified as candidates of the ER stress sensor transducer. One of these proteins, OsbZIP39, was characterized in this study. OsbZIP39 was shown to associate with microsomes as a membrane-integrated protein using the subcellular fractionation method. When the full length and a truncated form of OsbZIP39 without the TMD (OsbZIP39ΔC) was fused to green fluorescent protein (GFP) and transfected into rice protoplasts, the proteins were identified in the cytoplasm and nucleus, respectively. This suggests that OsbZIP39 may be converted into a soluble truncated form by proteolytic cleavage and subsequently translocated to the nucleus. Expression of OsbZIP39ΔC clearly activated the binding protein 1 (BiP1) promoter in a rice protoplast transient assay. Overexpression of OsbZIP39ΔC in stable transgenic rice also led to the up-regulation of several ER stress response genes including BiP1 and OsbZIP50 in the absence of ER stress. However, in the OsbZIP39ΔC-overexpressing line, OsbZIP50 mRNA did not undergo IRE1 (inositol-requiring protein 1)-mediated cytoplasmic splicing that is required for its activation. These data indicate that OsbZIP39 may be directly involved in the regulation of several ER stress response genes.