Characterization of two homologs of Ire1p, a kinase/endoribonuclease in yeast, in Arabidopsis thaliana.

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) elicits an ER-to-nucleus signaling pathway known as the unfolded protein response (UPR) in eukaryotes. In yeast, Ire1p, a kinase/endoribonuclease in the ER membrane, plays a key role in the UPR signaling. We isolated two cDNA homologs of IRE1 gene from Arabidopsis (AtIre1a, AtIre1b). The two IRE1 homologs were predicted to form a type I transmembrane protein structure and contain kinase/endoribonuclease domains at their C-terminal halves. The expressions of the two genes were detected in various organ tissues of the Arabidopsis plant. The C-terminal half of the AtIre1a protein showed in vitro autophosphorylation activity. However, we could not detect endoribonuclease activity of the AtIre1a protein when we used yeast HAC1 RNA as the substrate in vivo.