Transcriptional activator that may play a role in the unfolded protein response. Binds to the UPR element (UPRE) but not to CRE element. Preferentially binds DNA with to the consensus sequence 5'-T[GT]ACGT[GA][GT]-3' and has transcriptional activation activity from UPRE. Binds to NF-kappa-B site and has transcriptional activation activity from NF-kappa-B-containing regulatory elements (By similarity). Binds DNA as a dimer. Under ER stress the cleaved N-terminal cytoplasmic domain translocates into the nucleus. Controlled by regulated intramembrane proteolysis (RIP). Following ER stress a fragment containing the cytoplasmic transcription factor domain is released by proteolysis. The cleavage seems to be performed sequentially by site-1 and site-2 proteases (PS1 and PS2). PS1 cleavage may be suppressed by a determinant in the C-terminal region (By similarity). Belongs to the bZIP family. ATF subfamily.