Messenger RNA targeting to endoplasmic reticulum stress signalling sites

Abstract

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. It is shown that after 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′ UTR and is processes at these sites. Disruption of either Ire1 clustering or HAC1 mRNA recruitment impairs UPR signalling. Hence HAC1 mRNA is delivered to a site where it is processed thus ensuring that it is only translated when UPR is on. Deficiencies in the protein-folding capacity of the endoplasmic reticulum (ER) in all eukaryotic cells lead to ER stress and trigger the unfolded protein response (UPR)1,2,3. ER stress is sensed by Ire1, a transmembrane kinase/endoribonuclease, which initiates the non-conventional splicing of the messenger RNA encoding a key transcription activator, Hac1 in yeast or XBP1 in metazoans. In the absence of ER stress, ribosomes are stalled on unspliced HAC1 mRNA. The translational control is imposed by a base-pairing interaction between the HAC1 intron and the HAC1 5′ untranslated region4. After excision of the intron, transfer RNA ligase joins the severed exons5,6, lifting the translational block and allowing synthesis of Hac1 from the spliced HAC1 mRNA to ensue4. Hac1 in turn drives the UPR gene expression program comprising 7–8% of the yeast genome7 to counteract ER stress. Here we show that, on activation, Ire1 molecules cluster in the ER membrane into discrete foci of higher-order oligomers, to which unspliced HAC1 mRNA is recruited by means of a conserved bipartite targeting element contained in the 3′ untranslated region. Disruption of either Ire1 clustering or HAC1 mRNA recruitment impairs UPR signalling. The HAC1 3′ untranslated region element is sufficient to target other mRNAs to Ire1 foci, as long as their translation is repressed. Translational repression afforded by the intron fulfils this requirement for HAC1 mRNA. Recruitment of mRNA to signalling centres provides a new paradigm for the control of eukaryotic gene expression.