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

By D. Thuerauf, Marie C. Marcinko, Peter J. Belmont, C. Glembotski. Journal of Biological Chemistry, 2007

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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.

Subfields: Cell Biology, Molecular Biology

View Paper: https://www.semanticscholar.org/paper/eba453364c0e486a861ee7bd8bef2715a45ea197