Mammalian Transcription Factor ATF6 Is Synthesized as a Transmembrane Protein and Activated by Proteolysis in Response to Endoplasmic Reticulum Stress

Abstract

The unfolded protein response (UPR) controls the levels of molecular chaperones and enzymes involved in protein folding in the endoplasmic reticulum (ER). We recently isolated ATF6 as a candidate for mammalian UPR-specific transcription factor. We report here that ATF6 constitutively expressed as a 90-kDa protein (p90ATF6) is directly converted to a 50-kDa protein (p50ATF6) in ER-stressed cells. Furthermore, we showed that the most important consequence of this conversion was altered subcellular localization; p90ATF6 is embedded in the ER, whereas p50ATF6 is a nuclear protein. p90ATF6 is a type II transmembrane glycoprotein with a hydrophobic stretch in the middle of the molecule. Thus, the N-terminal half containing a basic leucine zipper motif is oriented facing the cytoplasm. Full-length ATF6 as well as its C-terminal deletion mutant carrying the transmembrane domain is localized in the ER when transfected. In contrast, mutant ATF6 representing the cytoplasmic region translocates into the nucleus and activates transcription of the endogenous GRP78/BiP gene. We propose that ER stress-induced proteolysis of membrane-bound p90ATF6 releases soluble p50ATF6, leading to induced transcription in the nucleus. Unlike yeast UPR, mammalian UPR appears to use a system similar to that reported for cholesterol homeostasis.

A highly conserved proapoptotic gene, IKIP, located next to the APAF1 gene locus, is regulated by p53

“…A possible explanation could be a cleavage event that results in a C-terminal fragment with altered localization. Cleavage of proteins at the ER membrane has been described; 23 however, for IKIP this has to await experimental verification.…”

Cellular response to endoplasmic reticulum stress: a matter of life or death

“…63 Instead, the expression of XBP-1 mRNA is regulated by the transcription factor ATF6, which is itself an important component of the ESR. 63,[66][67][68] The overall structure of ATF6 does not resemble Hac1p and ATF6 protein levels are not regulated by mRNA processing. 67 However, ATF6 does share significant sequence identity with Hac1p in its N-terminal basic region and ATF6 overexpression activates many targets of the mammalian UPR.…”

“…63,[66][67][68] The overall structure of ATF6 does not resemble Hac1p and ATF6 protein levels are not regulated by mRNA processing. 67 However, ATF6 does share significant sequence identity with Hac1p in its N-terminal basic region and ATF6 overexpression activates many targets of the mammalian UPR. [66][67][68][69] Like HAC1 and XBP-1, ATF6 is activated by ER stress via an unusual mechanism.…”

“…67 However, ATF6 does share significant sequence identity with Hac1p in its N-terminal basic region and ATF6 overexpression activates many targets of the mammalian UPR. [66][67][68][69] Like HAC1 and XBP-1, ATF6 is activated by ER stress via an unusual mechanism. ATF6 is a type II transmembrane cytoplasm Figure 1 Mammalian ESR signaling.…”

Rottlerin induces pro-apoptotic endoplasmic reticulum stress through the protein kinase C-δ-independent pathway in human colon cancer cells

“…Cells display various adaptive responses to relieve ER stress. The key players of the unfolded protein response (UPR) are three chronologically activated ER resident signaling proteins: protein kinaselike ER-resident kinase (PERK), inositol requiring 1 (Ire1) and activating transcription factor-6(ATF-6) [3][4][5][6]. Several pro-apoptotic factors and signaling pathways, including C/EBP homologous protein/growth arrest and DNA damage-inducible gene 153 (CHOP/GADD153), pro-apoptotic Bcl-2 family members, and caspase-12, have been shown to be involved in ER stress-induced cell death [7][8][9].…”