Precursor of the transcription factor form (Processed cyclic AMP-dependent transcription factor ATF-6 beta), which is embedded in the endoplasmic reticulum membrane. Endoplasmic reticulum stress promotes processing of this form, releasing the transcription factor form that translocates into the nucleus, where it activates transcription of genes involved in the unfolded protein response (UPR). Transcription factor that acts in the unfolded protein response (UPR) pathway by activating UPR target genes induced during ER stress. Binds DNA on the 5'-CCAC[GA]-3' half of the ER stress response element (ERSE) (5'-CCAATN(9)CCAC[GA]-3') when NF-Y is bound to ERSE. Homodimer and heterodimer with ATF6-alpha. The dimer interacts with the nuclear transcription factor Y (NF-Y) trimer through direct binding to NF-Y subunit C (NF-YC). Under ER stress the cleaved N-terminal cytoplasmic domain translocates into the nucleus. The basic domain functions as a nuclear localization signal. The basic leucine-zipper domain is sufficient for association with the NF-Y trimer and binding to ERSE. N-glycosylated. During unfolded protein response, a fragment of approximately 60 kDa containing the cytoplasmic transcription factor domain is released by proteolysis. The cleavage is probably performed sequentially by site-1 (MBTPS1, S1P) and site-2 (MBTPS2, S2P) proteases. Belongs to the bZIP family. ATF subfamily.