Senses unfolded proteins in the lumen of the endoplasmic reticulum via its N-terminal domain which leads to enzyme auto-activation. The active endoribonuclease domain splices bZIP60 mRNA to generate a new C-terminus, converting it into a potent unfolded-protein response transcriptional activator which then induces transcription of UPR target genes. Involved in organ growth regulation. Plays a role in plant immunity and abiotic stress responses