title: The nucleotide exchange factors Grp170 and Sil1 induce cholera toxin release from BiP to enable retrotranslocation.
abstract: Cholera toxin (CT) intoxicates cells by trafficking from the cell surface to the endoplasmic reticulum (ER), where the catalytic CTA1 subunit hijacks components of the ER-associated degradation (ERAD) machinery to retrotranslocate to the cytosol and induce toxicity. In the ER, CT targets to the ERAD machinery composed of the E3 ubiquitin ligase Hrd1-Sel1L complex, in part via the activity of the Sel1L-binding partner ERdj5. This J protein stimulates BiP's ATPase activity, allowing BiP to capture the toxin. Presumably, toxin release from BiP must occur before retrotranslocation. Here, using loss-and gain-of-function approaches coupled with binding studies, we demonstrate that the ER-resident nucleotide exchange factors (NEFs) Grp170 and Sil1 induce CT release from BiP in order to promote toxin retrotranslocation. In addition, we find that after NEF-dependent release from BiP, the toxin is transferred to protein disulfide isomerase; this ER redox chaperone is known to unfold CTA1, which allows the toxin to cross the Hrd1-Sel1L complex. Our data thus identify two NEFs that trigger toxin release from BiP to enable successful retrotranslocation and clarify the fate of the toxin after it disengages from BiP.
authors: Department of Cell and Developmental Biology, University of Michigan Medical School, Ann Arbor, MI 48103.
authors: Jeffrey M
authors: JM
authors: Williams
authors: Department of Cell and Developmental Biology, University of Michigan Medical School, Ann Arbor, MI 48103.
authors: Takamasa
authors: T
authors: Inoue
authors: Department of Cell and Developmental Biology, University of Michigan Medical School, Ann Arbor, MI 48103.
authors: Grace
authors: G
authors: Chen
authors: Department of Cell and Developmental Biology, University of Michigan Medical School, Ann Arbor, MI 48103 btsai@umich.edu.
authors: Billy
authors: B
authors: Tsai
mesh_terms: D002772:Cholera Toxin / Q000378:metabolism* / Q000494:pharmacology; D000091342:Endoplasmic Reticulum Chaperone BiP; D018870:Endoplasmic Reticulum, Rough / Q000187:drug effects / Q000378:metabolism*; D060746:Endoplasmic Reticulum-Associated Degradation / Q000187:drug effects; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism*; D006360:Heat-Shock Proteins / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D019704:Protein Disulfide-Isomerases / Q000378:metabolism
chemical_list: D000091342:Endoplasmic Reticulum Chaperone BiP; D020662:Guanine Nucleotide Exchange Factors; D018840:HSP70 Heat-Shock Proteins; D006360:Heat-Shock Proteins; C418483:SIL1 protein, human; C050087:oxygen-regulated proteins; D002772:Cholera Toxin; D019704:Protein Disulfide-Isomerases