Functional analysis of the mammalian RNA ligase for IRE1 in the unfolded protein response

Abstract

The unfolded protein response (UPR) is a conserved signalling pathway activated on the accumulation of unfolded proteins within the endoplasmic reticulum (ER), termed ER stress. Upon ER stress, HAC1/XBP1 undergoes exon/intron-specific excision by inositol requiring enzyme 1 (IRE1) to remove an intron and liberate the 5′ and 3′ exons. In yeast, the 5′ and 3′ HAC1 exons are subsequently ligated by tRNA ligase (Rlg1p), whereas XBP1 ligation in mammalian cells is catalysed by a recently identified ligase, RtcB. In the present study, RNA ligase activity of the human RtcB (hRtcB) involved in the unconventional splicing of XBP1/HAC1 mRNA was explored in an rlg1-100 mutant yeast strain. Distinct from Escherichia coli RtcB and Rlg1p, expression of hRtcB alone inefficiently complemented HAC1/XBP1 splicing and the hRtcB cofactor (archease) was required to promote enzymatic activity of hRtcB to catalyse RNA ligation.

A Molecular Mechanism for Turning Off IRE1α Signaling during Endoplasmic Reticulum Stress

“…Once activated, IRE1a mediates nonconventional splicing of XBP1 mRNA (Calfon et al, 2002;Yoshida et al, 2001), which is recruited to the Sec61 translocon through its ribosome nascent chain (Plumb et al, 2015;Yanagitani et al, 2011). The cleaved fragments of XBP1 mRNA are subsequently ligated by the RtcB tRNA ligase (Jurkin et al, 2014;Kosmaczewski et al, 2014;Lu et al, 2014) with its co-factor archease (Poothong et al, 2017). The spliced XBP1 mRNA is translated into a functional transcription factor, XBP1s, which induces the expression of chaperones, quality control factors, and protein translocation components (Lee et al, 2003).…”

The Sec61 translocon limits IRE1α signaling during the unfolded protein response

“…Efficient cleavage of XBP1u mRNA requires an interaction between IRE1α and the Sec61 translocon as well as the SRP pathway-mediated recruitment of XBP1u mRNA to the Sec61 translocon ( Plumb et al, 2015 ; Kanda et al, 2016 ). Subsequently, the cleaved fragments of XBP1 mRNA are ligated by the RtcB tRNA ligase ( Lu et al, 2014 ; Jurkin et al, 2014 ; Kosmaczewski et al, 2014 ) with its co-factor archease ( Poothong et al, 2017 ). The spliced XBP1 mRNA is translated into an active transcription factor, XBP1s, which induces UPR genes to alleviate ER stress ( Lee et al, 2003 ; Acosta-Alvear et al, 2007 ).…”

Splicing HAC1/XBP1 mRNAs in Cytoplasm: The Non-Conventional mRNA Splicing Reaction in the Unfolded Protein Response

“…In professional secretory cells, like the plasma cells, depleting RTCB decreases its ER protein folding capacity and, subsequently, its antibody production [87]. When simultaneously expressed in S. cerevisiae cells, mammalian RTCB and archease can catalyze the ligation reaction of yeast HAC1 mRNA exons [91]. Interestingly, a recent study showed that a tyrosine phosphorylation on RTCB reduces the enzyme's efficiency to ligate XBP1 mRNA exons, suggesting additional layers of regulation [92].…”