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

TLDR

The unfolded protein response regulates ER chaperones and enzymes, and in mammals it resembles the cholesterol‑homeostasis system rather than the yeast pathway. The study aims to show that ER‑stress–induced proteolysis of membrane‑bound p90ATF6 releases soluble p50ATF6, which then activates nuclear transcription. ATF6 is a type‑II transmembrane protein whose N‑terminal bZIP domain faces the cytoplasm; upon ER stress it is cleaved to release a soluble p50 fragment that translocates to the nucleus and activates transcription. We found that ATF6 is synthesized as a 90‑kDa ER‑membrane protein that is proteolytically cleaved to a 50‑kDa nuclear form, and that the cytoplasmic domain alone is sufficient to relocate to the nucleus and activate the GRP78/BiP gene.

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.