Precursor of the transcription factor form (Processed cyclic AMP-dependent transcription factor ATF-6 alpha), which is embedded in the endoplasmic reticulum membrane (PubMed:10564271, PubMed:11158310, PubMed:11779464). 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) (PubMed:10564271, PubMed:11158310, PubMed:11779464). Transcription factor that initiates the unfolded protein response (UPR) during endoplasmic reticulum stress by activating transcription of genes involved in the UPR (PubMed:10564271, PubMed:11163209, PubMed:11158310, PubMed:11779464). Binds DNA on the 5'-CCAC[GA]-3'half of the ER stress response element (ERSE) (5'-CCAAT-N(9)-CCAC[GA]-3') and of ERSE II (5'-ATTGG-N-CCACG-3') (PubMed:10564271, PubMed:11158310, PubMed:11779464). Binding to ERSE requires binding of NF-Y to ERSE. Could also be involved in activation of transcription by the serum response factor (PubMed:10564271, PubMed:11158310, PubMed:11779464). May play a role in foveal development and cone function in the retina (PubMed:26029869). Interacts with XBP1 isoform 2; the interaction occurs in a ER stress-dependent manner (PubMed:17765680). Interacts with LACC1 (PubMed:31875558). Interacts with THBS4 (via EGF-like 3; calcium-binding domain) which facilitates its processing, activation and nuclear translocation (PubMed:22682248). Interacts (via lumenal domain) with THBS1 (By similarity). Homodimer and heterodimer with ATF6-beta. The dimer interacts with the nuclear transcription factor Y (NF-Y) trimer through direct binding to NF-Y subunit C (NF-YC). Interacts also with the transcription factors GTF2I, YY1 and SRF. Translocates from the endoplasmic reticulum to the Golgi, where it is processed. Under ER stress the cleaved N-terminal cytoplasmic domain translocates into the nucleus (PubMed:10564271, PubMed:12782636). THBS4 promotes its nuclear shuttling (By similarity). Ubiquitous. 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. During unfolded protein response, a fragment of approximately 50 kDa containing the cytoplasmic transcription factor domain is released by proteolysis. The cleavage seems to be performed sequentially by site-1 (MBTPS1, S1P) and site-2 (MBTPS2, S2P) proteases. N-glycosylated. The glycosylation status may serve as a sensor for ER homeostasis, resulting in ATF6 activation to trigger the unfolded protein response (UPR). Ubiquitinated by RNF186 at Lys-152, which is required for pattern recognition receptor-induced unfolded protein response-associated outcomes. The disease is caused by variants affecting the gene represented in this entry. Belongs to the bZIP family. ATF subfamily.