Transcriptional activator. Binds the cAMP response element (CRE). Activates transcription through box-B element and CRE. Seems to function synergistically with atf6 (By similarity). Regulates FGF21 transcription (By similarity). Binds DNA as a dimer. Controlled by regulated intramembrane proteolysis (RIP). 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 (By similarity). Belongs to the bZIP family. ATF subfamily.