Making ends meet: a role of RNA ligase RTCB in unfolded protein response

The unfolded protein response (UPR) monitors the protein folding capacity of the endoplasmic reticulum. One of the UPR branches includes an unusual cytoplasmic splicing reaction leading to removal of an intron from an mRNA encoding a key UPR transcription factor. The cleavage step of the process is well characterized in both yeast and animals, but the animal enzyme responsible for exon ligation has remained a mystery. Recent reports, including a paper in this issue of The EMBO Journal, identify RTCB as the RNA ligase operating during UPR in mammals and C. elegans.