The unfolded protein response signals through high-order assembly of Ire1

Abstract

Aberrant folding of proteins in the endoplasmic reticulum activates the bifunctional transmembrane kinase/endoribonuclease Ire1. Ire1 excises an intron from HAC1 messenger RNA in yeasts and Xbp1 messenger RNA in metozoans encoding homologous transcription factors. This non-conventional mRNA splicing event initiates the unfolded protein response, a transcriptional program that relieves the endoplasmic reticulum stress. Here we show that oligomerization is central to Ire1 function and is an intrinsic attribute of its cytosolic domains. We obtained the 3.2-Å crystal structure of the oligomer of the Ire1 cytosolic domains in complex with a kinase inhibitor that acts as a potent activator of the Ire1 RNase. The structure reveals a rod-shaped assembly that has no known precedence among kinases. This assembly positions the kinase domain for trans-autophosphorylation, orders the RNase domain, and creates an interaction surface for binding of the mRNA substrate. Activation of Ire1 through oligomerization expands the mechanistic repertoire of kinase-based signalling receptors. The accumulation of misfolded proteins activates the unfolded protein response in the endoplasmic reticulum. The transmembrane protein Ire1 is a central player in this pathway, acting as a kinase and an endoribonuclease. It excises an intron on HAC1 mRNA resulting in translation of the transcription factor Hac1 that in turn activates target genes. This issue reports two studies on Ire1. Korennykh et al. solve the crystal structure of Ire1 kinase and show that it spontaneously assembles into a rod-shaped oligomer. This positions the kinase domains for trans-phosphorylation, orders the RNase domains and creates an interaction site for mRNA substrate binding. Aragón et al. show that on activation, Ire1 molecules cluster into discrete foci containing high-order oligomers on the endoplasmic reticulum membrane. HAC1 mRNA is recruited to these foci by means of a sequence in its 3′ untranslated region and is processed at these sites. In this way the HAC1 mRNA is delivered to a site where it is processed, ensuring that it is translated only when the unfolded protein response is on. Accumulation of misfolded proteins results in the activation of the unfolded protein response (UPR) in the endoplasmic reticulum. Ire1 is important in this pathway and functions as a kinase and endoribonuclease. This paper solves the crystal structure of Ire1 kinase and shows that it undergoes spontaneous assembly into a rod-shaped oligomer. This arrangement positions the kinase domains for trans-autophosphorylation, orders the RNase domains and creates an interaction site for mRNA substrate binding.