Functions as a transcription factor during endoplasmic reticulum (ER) stress by regulating the unfolded protein response (UPR). Required for cardiac myogenesis and hepatogenesis during embryonic development, and the development of secretory tissues such as exocrine pancreas and salivary gland. Involved in terminal differentiation of B lymphocytes to plasma cells and production of immunoglobulins. Modulates the cellular response to ER stress in a PIK3R-dependent manner. Binds to the cis-acting X box present in the promoter regions of major histocompatibility complex class II genes. Involved in VEGF-induced endothelial cell (EC) proliferation and retinal blood vessel formation during embryonic development but also for angiogenesis in adult tissues under ischemic conditions. Functions also as a major regulator of the UPR in obesity-induced insulin resistance and type 2 diabetes for the management of obesity and diabetes prevention. Acts as a weak transcriptional factor. Together with HDAC3, contributes to the activation of NFE2L2-mediated HMOX1 transcription factor gene expression in a PI(3)K/mTORC2/Akt-dependent signaling pathway leading to EC survival under disturbed flow/oxidative stress. Binds to the ER stress response element (ERSE) upon ER stress. Binds to the consensus 5'-GATGACGTG[TG]N(3)[AT]T-3' sequence related to cAMP responsive element (CRE)-like sequences. Associates preferentially to the HDAC3 gene promoter region in a static flow-dependent manner. Binds to the CDH5/VE-cadherin gene promoter region. Isoform 1 interacts with HM13. Isoform 1 interacts with RNF139; the interaction induces ubiquitination and degradation of isoform 1. Isoform 1 interacts (via luminal domain) with DERL1; the interaction obviates the need for ectodomain shedding prior HM13/SPP-mediated XBP1 isoform 1 cleavage. Isoform 1 interacts with HDAC3 and AKT1; the interactions occur in endothelial cell (EC) under disturbed flow. Isoform 1 interacts with the oncoprotein FOS. Interacts with SIRT1. Colocalizes with ERN1 and KDR in the endoplasmic reticulum in endothelial cells in a vascular endothelial growth factor (VEGF)-dependent manner. Colocalizes in the nucleus with SIRT1. Shows no preferential localization to either the nucleus or the cytoplasm. Shuttles between the nucleus and the cytoplasm in a CRM1-dependent manner. Localizes predominantly at the endoplasmic reticulum membrane as a membrane-spanning protein; whereas may be only marginally localized on the cytosolic side of the ER membrane as a peripheral membrane. Additional isoforms seem to exist. Isoform 1 N-terminus domain is necessary for nuclear localization targeting. Isoform 1 C-terminus domain confers localization to the cytoplasm and is sufficient to impose rapid degradation. Isoform 1 N-terminus and C-terminus regions are necessary for DNA-binding and weak transcriptional activity, respectively. Isoform 1 is ubiquitinated, leading to proteasome-mediated degradation in response to ER stress. X-box-binding protein 1, cytoplasmic form and luminal form are produced by intramembrane proteolytic cleavage of ER membrane-anchored isoform 1 triggered by HM13/SPP in a DERL1-RNF139-dependent and VCP/p97-independent manner. X-box-binding protein 1, luminal form is ubiquitinated leading to proteasomal degradation. Acetylated by EP300; acetylation positively regulates the transcriptional activity of XBP1. Deacetylated by SIRT1; deacetylation negatively regulates the transcriptional activity of XBP1. Belongs to the bZIP family.