XBP1

X-box binding protein 1, also known as XBP1, is a human gene.

Function
The transcription factor X-box binding protein 1 (XBP-1) is a bZIP transcription factor first identified by its ability to bind to the x-box, a conserved transcriptional element, in the human leukocyte antigen (HLA) DR alpha promoter. XBP-1 is also essential for plasma cell differentiation. This differentiation requires not only the expression of XBP-1 but the expression of the spliced isoform of XBP-1s. This protein has also been identified as a cellular transcription factor that binds to an enhancer in the promoter of the T cell leukemia virus type 1 promoter. The generation of XBP-1s during plasma cell differentiation also seems to be the cue for Kaposi's sarcoma-associated herpesvirus and Epstein Barr virus reactivation from latency.

XBP-1 is upregulated as part of the ER stress response, the unfolded protein response (UPR). This increase in transcription requires an ER stress response consensus binding element in the promoter. XBP-1u is ubiquitously expressed but under conditions of ER-stress, the XBP-1u mRNA is processed by IRE1. Activated IRE1 oligomerises and activates its ribonuclease domain through autophosphorylation. Because the lumen of the ER is continuous with the perinuclear space, the activated ribonuclease domains can penetrate the inner leaflet of the nuclear envelope. Within the nucleus, activated IRE1 catalyses the excision of a 26 nucleotide unconventional intron from XBP-1 mRNA, in a manner mechanistically similar to pre-tRNA splicing. Removal of this intron causes a frame shift in the XBP-1 coding sequence resulting in the translation of a 371 amino acid, 54 kDa, XBP-1s isoform rather than the 261 amino acid, 33 kDa, XBP-1u isoform.

Finally, a pseudogene has been identified and localized to chromosome 5.