Endoplasmic Reticulum Stress-Induced Formation of Transcription Factor Complex ERSF Including NF-Y (CBF) and Activating Transcription Factors 6a and 6b That Activates the Mammalian Unfolded Protein Response

The levels of molecular chaperones and folding enzymes in the endoplasmic reticulum (ER) are controlled by a transcriptional induction process termed the unfolded protein response (UPR). The mammalian UPR is mediated by the cis-acting ER stress response element (ERSE), the consensus sequence of which is CCAATN9-CCACG. We recently proposed that ER stress response factor (ERSF) binding to ERSE is a heterologous protein complex consisting of the constitutive component NF-Y (CBF) binding to CCAAT and an inducible component binding to CCACG and identified the basic leucine zipper-type transcription factors ATF6a and ATF6b as inducible components of ERSF. ATF6a and ATF6b produced by ER stress-induced proteolysis bind to CCACG only when CCAAT is bound to NF-Y, a heterotrimer consisting of NF-YA, NF-YB, and NF-YC. Interestingly, the NF-Y and ATF6 binding sites must be separated by a spacer of 9 bp. We describe here the basis for this strict requirement by demonstrating that both ATF6a and ATF6b physically interact with NF-Y trimer via direct binding to the NF-YC subunit. ATF6a and ATF6b bind to the ERSE as a homoor heterodimer. Furthermore, we showed that ERSF including NF-Y and ATF6a and/or b and capable of binding to ERSE is indeed formed when the cellular UPR is activated. We concluded that ATF6 homoor heterodimers recognize and bind directly to both the DNA and adjacent protein NF-Y and that this complex formation process is essential for transcriptional induction of ER chaperones.