title: Protein disulphide-isomerase reduces ricin to its A and B chains in the endoplasmic reticulum.
abstract: Cells expressing ricin B chain within the secretory pathway are significantly more resistant to intoxication by ricin holotoxin but not to other cytotoxins that exploit similar endocytic routes to the cytosol. Furthermore, cells expressing the related B chain of abrin are protected against both incoming abrin and ricin. These phenotypes can be correlated with the abilities of the respective B chains to form disulphide-linked A-B holotoxins, since abrin B chain forms heterodimers with either abrin or ricin A chains, whereas ricin B chain forms heterodimers with ricin A chain only. In the ricin B-expressing cells, this newly made lectin disappears with biphasic kinetics comprising a retention phase followed by slow turnover and disposal after disengagement from calnexin cycle components. Interference with ricin cytotoxicity occurs during the early retention phase when ricin B chain is associated with PDI (protein disulphide-isomerase). The data show that retrotranslocation of incoming toxin is impeded by PDI-catalysed formation of heterodimers between endogenous B and A chains derived from reduced holotoxin, thus proving that reduction of ricin occurs in the endoplasmic reticulum. In contrast with other toxins, ricin does not appear to require either proteolytic cleavage or unfolding for PDI-catalysed reduction.
authors.affiliation: Molecular Cell Biology, Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Spooner
authors.forename: Peter D
authors.initials: PD
authors.lastname: Watson
authors.forename: Catherine J
authors.initials: CJ
authors.lastname: Marsden
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Smith
authors.forename: Katherine A H
authors.initials: KA
authors.lastname: Moore
authors.forename: Jonathon P
authors.initials: JP
authors.lastname: Cook
authors.forename: J Michael
authors.initials: JM
authors.lastname: Lord
authors.forename: Lynne M
authors.initials: LM
authors.lastname: Roberts
mesh_terms: D000036:Abrin / Q000494:pharmacology; D001692:Biological Transport; D019281:Dimerization; D004220:Disulfides / Q000378:metabolism; D004705:Endocytosis; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D010084:Oxidation-Reduction; D010447:Peptide Hydrolases / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D019704:Protein Disulfide-Isomerases / Q000378:metabolism*; D017510:Protein Folding; D020836:Protein Structure, Quaternary; D012276:Ricin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D013438:Sulfhydryl Compounds / Q000737:chemistry / Q000378:metabolism
chemical_list: D004220:Disulfides; D013438:Sulfhydryl Compounds; D000036:Abrin; D012276:Ricin; D010447:Peptide Hydrolases; D046988:Proteasome Endopeptidase Complex; D019704:Protein Disulfide-Isomerases