ATF6 as a transcription activator of the endoplasmic reticulum stress element: Thapsigargin stress-induced changes and synergistic interactions with NF-Y and YY1

ATF6, a member of the leucine zipper protein family, can constitutively ind uce the promoter of glucose-regulated protein (grp) genes through activatio n of the endoplasmic reticulum (ER) stress element (ERSE). To understand th e mechanism of grp78 induction by ATF6 in cells subjected to ER calcium dep letion stress mediated by thapsigargin (Tg) treatment, we discovered that A TF6 itself undergoes Tg stress induced changes. In nonstressed cells, ATF6, which contains a putative short transmembrane domain, is primarily associa ted with the perinuclear region. Upon Tg stress, the ATF6 protein level dro pped initially but quickly recovered with the additional appearance of a fa ster-migrating form. This ne cv form of ATF6 was recovered as soluble nucle ar protein by biochemical fractionation, correlating with enhanced nuclear localization of ATF6 as revealed by immunofluorescence. Optimal ATF6 stimul ation requires at least two copies of the ERSE and the integrity of the tri partite structure of the ERSE. Of primary importance is a functional NF-Y c omplex and a high-affinity NF-Y binding site that confers selectivity among different ERSEs for ATF6 inducibility. In addition, we showed that YY1 int eracts with ATF6 and in Tg-treated cells can enhance ATF6 activity. The ERS E stimulatory activity of ATF6 exhibits properties distinct from those of h uman Ire1p, an upstream regulator of the mammalian unfolded protein respons e. The requirement for a high-affinity NF-Y site for ATF6 but not human Ire 1p activity suggests that they stimulate the ERSE through diverse pathways.