Molecular characterization of two arabidopsis Ire1 homologs, endoplasmic reticulum-located transmembrane protein kinases

A major response of eukaryotic cells to the presence of unfolded proteins i n the lumen of the endoplasmic reticulum (ER) is to activate genes that enc ode ER-located molecular chaperones, such as the binding protein. This resp onse, called the unfolded protein response, requires the transduction of a signal from the ER to the nucleus. In yeast (Saccharomyces cerevisiae) and mammalian cells, an ER-located transmembrane receptor protein kinase/ribonu clease called Ire1, with a sensor domain in the lumen of the ER, is the fir st component of this pathway. Here, we report the cloning and derived amino acid sequences of AtIre1-1 and AtIre1-2, two Arabidopsis homologs of Ire1. The two proteins are located in the perinuclear ER (based on heterologous expression of fusions with green fluorescent protein). The expression patte rns of the two genes (using, beta -glucuronidase fusions) arc nearly nonove rlapping. We also demonstrate functional complementation of the sensor doma ins of the two proteins in yeast and show that the Ire1-2 protein is capabl e of autotransphosphorylation. These and other findings are discussed in re lation to the involvement of these genes in unfolded protein response signa ling in plants.