The endoribonuclease activity of mammalian IRE1 autoregulates its mRNA andis required for the unfolded protein response

The unfolded protein response (UPR) is a signal transduction pathway that i s activated by the accumulation of unfolded proteins in the endoplasmic ret iculum (ER). In Saccharomyces cerevisiae the ER transmembrane receptor, Ire 1p, transmits the signal to the nucleus culminating in the transcriptional activation of genes encoding an adaptive response. Yeast Ire1p requires bot h protein kinase and site-specific endoribonuclease (RNase) activities to s ignal the UPR. In mammalian cells, two homologs, Ire1 alpha and Ire1 beta, are implicated in signaling the UPR. To elucidate the RNase requirement for mammalian Ire1 function, we have identified five amino acid residues withi n IRE1 alpha that are essential for RNase activity but not kinase activity. These mutants were used to demonstrate that the RNase activity is required for UPR activation by IRE1 alpha and IRE1 beta. In addition, the data supp ort that IRE1 RNase is activated by dimerization-induced hans-autophosphory lation and requires a homodimer of catalytically functional RNase domains. Finally, the RNase activity of wild-type IRE1 alpha down-regulates hIre1 al pha mRNA expression by a novel mechanism involving cis-mediated IRE1 alpha -dependent cleavage at three specific sites within the 5' end of Ire1 alpha mRNA.