Luman/CREB3 induces transcription of the endoplasmic reticulum (ER) stress response protein Herp through an ER stress response element.

Luman/CREB3 (also called LZIP) is an endoplasmic reticulum (ER) membrane-bound transcription factor which is believed to undergo regulated intramembrane proteolysis in response to cellular cues. We previously found that Luman activates transcription from the unfolded protein response element. Here we report the identification of Herp, a gene involved in ER stress-associated protein degradation (ERAD), as a direct target of Luman. We found that Luman was transcriptionally induced and proteolytically activated by the ER stress inducer thaspsigargin. Overexpression of Luman activated transcription of cellular Herp via ER stress response element II (ERSE-II; ATTGG-N-CCACG) in the promoter region. Mutagenesis studies and chromatin immunoprecipitation assays showed that Luman physically associates with the Herp promoter, specifically the second half-site (CCACG) of ERSE-II. Luman was also necessary for the full activation of Herp during the ER stress response, since Luman small interfering RNA knockdown or functional repression by a dominant negative mutant attenuated Herp gene expression. Like Herp, overexpression of Luman protected cells against ER stress-induced apoptosis. With Luman structurally similar to ATF6 but resembling XBP1 in DNA-binding specificities, we propose that Luman is a novel factor that plays a role in ERAD and a converging point for various signaling pathways channeling through the ER.

Cooperative interaction of Zhangfei and ATF4 in transactivation of the cyclic AMP response element.

Zhangfei (ZF) is a basic region-leucine zipper protein that has been implicated in herpesvirus infection cycle and related cellular processes. Here we show both in vivo and in vitro data demonstrating that ZF is a novel cellular binding partner of activating transcription factor 4 (ATF4) (or CREB2). We found that ZF competed with ATF4 to form ATF4-ZF heterodimeric complexes through the bZIP regions. ZF enhanced ATF4 binding to the cAMP response element (CRE), and augmented activation of a CRE reporter by ATF4, in response to MEK1 activation. These results suggest an important role of ZF in the MEK1-ATF4 signaling pathway.

Identification and characterization of the DNA-binding properties of a Zhangfei homologue in Japanese pufferfish, Takifugu rubripes.

Zhangfei is a basic region-leucine zipper (bZIP) transcription factor identified through its interaction with a herpesvirus-related host cell factor HCF1 (C1). Unlike most bZIP proteins, the mammalian Zhangfei protein does not bind DNA as homodimers. It is believed due to the absence of an asparagine residue in the basic region, which forms the DNA-recognition motif, NxxAAxxCR, in all bZIP proteins. Here, we report the identification and characterization of a novel Zhangfei homologue in Takifugu rubripes, which has an intact DNA-recognition motif by sequence analysis. We found that the pufferfish Zhangfei (pZF) appeared to have all the functional domains known in human Zhangfei, including the conserved HCF1-binding motif; however, pZF did not appear to bind DNA either. These findings suggest that the distinct property of the Zhangfei basic region is conserved during the evolution of vertebrates and that Zhangfei requires interaction with other proteins to regulate transcription from target promoters.

Luman is capable of binding and activating transcription from the unfolded protein response element.

Luman (or LZIP, CREB3) is a transcription factor with an endoplasmic reticulum (ER)-transmembrane domain. Due to its structural similarities with ATF6, it is thought that Luman might also be involved in cellular stress responses. Here we report that Luman can bind and activate transcription from the consensus unfolded protein response element (UPRE). Mutations that disrupted the binding of Luman to the UPREs impaired its ability to activate transcription from these sites. Overexpression of Luman stimulated transcription of EDEM, a downstream effector of the mammalian unfolded protein response involved in ER-associated degradation (ERAD). Unlike ATF6, however, Luman was not activated by proteolytic cleavage in response to endoplasmic reticulum stressors such as tunicamycin and thapsigargin. These results suggest that the activation of ERAD by Luman is likely through a pathway different from the common ER stress response, and that additional factor(s) are required for the activation of this Luman-mediated pathway.