Identification of TFII-I as the endoplasmic reticulum stress response element binding factor ERSF: Its autoregulation by stress and interaction with ATF6

When mammalian cells are subjected to stress targeted to the endoplasmic re ticulum (ER), such as depletion of the ER Ca2+ store, the transcription of a family of glucose regulated protein (GRP) genes encoding ER chaperones is induced. The GRP promoters contain multiple copies of the ER stress respon se element (ERSE), consisting of a unique tripartite structure, CCAAT(N-9)C CACG. Within a subset of mammalian ERSEs, N-9 represents a GC-rich sequence of 9 bp that is conserved across species. A novel complex (termed ERSF) ex hibits enhanced binding to the ERSE of the grp78 and ERp72 promoters using HeLa nuclear extracts prepared from ER-stressed cells. Optimal binding of E RSF to ERSE and maximal ERSE mediated stress inducibility require the conse rved GGC motif within the 9 bp region. Through chromatographic purification and subsequent microsequencing, we have identified ERSF as TFII-I. Whereas TFII-I remains predominantly nuclear in both nontreated NIH 3T3 cells and cells treated with thapsigargin (Tg), a potent inducer of the GRP stress re sponse through depletion of the ER Ca2+ store, the level of TFII-I transcri pt was elevated in Tg-stressed cells, correlating with an increase in TFII- I protein level in the nuclei of Tg -stressed cells. Purified recombinant T FII-I isoforms bind directly to the ERSEs of grp78 and ERp72 promoters. The stimulation of ERSE-mediated transcription by TFII-I requires the consensu s tyrosine phosphorylation site of TFII-I and the GGC sequence motif of the ERSE, We further discovered that TFII-I is an interactive protein partner of ATF6 and that optimal stimulation of ERSE by ATF6 requires TFII-I.