Effects of the Isoform-specific Characteristics of ATF6α and ATF6β on Endoplasmic Reticulum Stress Response Gene Expression and Cell Viability*

The endoplasmic reticulum (ER)-transmembrane proteins, ATF6α and ATF6β, are cleaved during the ER stress response (ERSR). The resulting N-terminal fragments (N-ATF6α and N-ATF6β) have conserved DNA-binding domains and divergent transcriptional activation domains. N-ATF6α and N-ATF6β translocate to the nucleus, bind to specific regulatory elements, and influence expression of ERSR genes, such as glucose-regulated protein 78 (GRP78), that contribute to resolving the ERSR, thus, enhancing cell viability. We previously showed that N-ATF6α is a rapidly degraded, strong transcriptional activator, whereas β is a slowly degraded, weak activator. In this study we explored the molecular basis and functional impact of these isoform-specific characteristics in HeLa cells. Mutants in the transcriptional activation domain or DNA-binding domain of N-ATF6α exhibited loss of function and increased expression, the latter of which suggested decreased rates of degradation. Fusing N-ATF6α to the mutant estrogen receptor generated N-ATF6α-MER, which, without tamoxifen exhibited loss-of-function and high expression, but in the presence of tamoxifen N-ATF6α-MER exhibited gain-of-function and low expression. N-ATF6β conferred loss-of-function and high expression to N-ATF6α, suggesting that ATF6β is an endogenous inhibitor of ATF6α. In vitro DNA binding experiments showed that recombinant N-ATF6β inhibited the binding of recombinant N-ATF6α to an ERSR element from the GRP78 promoter. Moreover, siRNA-mediated knock-down of endogenous ATF6β increased GRP78 promoter activity and GRP78 gene expression, as well as augmenting cell viability. Thus, the relative levels of ATF6α and -β, may contribute to regulating the strength and duration of ATF6-dependent ERSR gene induction and cell viability.