TREB5, XBP2, X-box-binding protein 1, XBP-1, Tax-responsive element-binding protein 5
Following protein synthesis, secretory, intra-organellar, and transmembrane proteins translocate into the endoplasmic reticulum (ER) where they are post-translationally modified and properly folded. The accumulation of unfolded proteins within the ER triggers an adaptive mechanism known as the unfolded protein response (UPR) that counteracts compromised protein folding. The transmembrane serine/threonine kinase IRE1, originally identified in Saccharomyces cerevisiae, is a proximal sensor for the UPR that transmits the unfolded protein signal across the ER membrane. The human homolog IRE1alpha was later identified and is ubiquitously expressed in human tissues. Upon activation of the unfolded protein response, IRE1alpha splices X-box binding protein 1 (XBP-1) mRNA through an unconventional mechanism using its endoribonuclease activity. This reaction converts XBP-1 from an unspliced XBP-1u isoform to the spliced XBP-1s isoform, which is a potent transcriptional activator that induces expression of many UPR responsive genes.
