The mammalian unfolded protein response (UPR) protects the cell against the stress
of misfolded proteins in the endoplasmic reticulum (ER). We have investigated here
the contribution of the UPR transcription factors XBP-1, ATF6alpha, and ATF6beta to
UPR target gene expression. Gene profiling of cell lines lacking these factors yielded
several XBP-1-dependent UPR target genes, all of which appear to act in the ER. These
included the DnaJ/Hsp40-like genes, p58(IPK), ERdj4, and HEDJ, as well as EDEM, protein
disulfide isomerase-P5, and ribosome-associated membrane protein 4 (RAMP4), whereas
expression of BiP was only modestly dependent on XBP-1. Surprisingly, given previous
reports that enforced expression of ATF6alpha induced a subset of UPR target genes,
cells deficient in ATF6alpha, ATF6beta, or both had minimal defects in upregulating
UPR target genes by gene profiling analysis, suggesting the presence of compensatory
mechanism(s) for ATF6 in the UPR. Since cells lacking both XBP-1 and ATF6alpha had
significantly impaired induction of select UPR target genes and ERSE reporter activation,
XBP-1 and ATF6alpha may serve partially redundant functions. No UPR target genes that
required ATF6beta were identified, nor, in contrast to XBP-1 and ATF6alpha, did the
activity of the UPRE or ERSE promoters require ATF6beta, suggesting a minor role for
it during the UPR. Collectively, these results suggest that the IRE1/XBP-1 pathway
is required for efficient protein folding, maturation, and degradation in the ER and
imply the existence of subsets of UPR target genes as defined by their dependence
on XBP-1. Further, our observations suggest the existence of additional, as-yet-unknown,
key regulators of the UPR.