Transcription factor that acts during endoplasmic reticulum stress by activating unfolded protein response target genes. Binds DNA on the 5-CCAC[GA]-3half of the ER stress response element (ERSE) (5-CCAAT-N(9)-CCAC[GA]-3) and of ERSE II (5-ATTGG-N-CCACG-3). Binding to ERSE requires binding of NF-Y to ERSE. Could also be involved in activation of transcription by the serum response factor.