title: Ricin A chain can transport unfolded dihydrofolate reductase into the cytosol.
abstract: Ricin is a heterodimeric protein toxin. The ricin A chain is able to cross the membrane of intracellular compartments to reach the cytosol where it catalytically inactivates protein synthesis. It is linked via a disulfide bond to the B chain, a galactose-specific lectin, which allows ricin binding at the cell surface and endocytosis. To examine the potential of ricin A to carry proteins into the cytosol and the requirement for unfolding of the passenger protein, we connected mouse dihydrofolate reductase (DHFR) to ricin A by gene fusion via a spacer peptide. DHFR-ricin A expressed in Escherichia coli displayed the biological activities of the parent proteins and associated quantitatively with ricin B to form DHFR-ricin. The resulting toxin was highly cytotoxic to cells (4-8-fold less than recombinant ricin). DHFR-ricin cytotoxicity was inhibited by methotrexate, a DHFR inhibitor stabilizing DHFR-ricin A in a folded conformation. The DHFR moiety of DHFR ricin bound to the plasma membrane. Although methotrexate prevented this binding, it did not significantly affect DHFR-ricin endocytosis, which proceeded via ricin B chain. Intoxication kinetics data and a cell-free translocation assay demonstrated that protection of cells from DHFR-ricin cytotoxicity resulted from a selective inhibition by methotrexate of DHFR-ricin A translocation. We conclude that ricin A is a potential carrier of proteins to the cytosol, provided that the passenger protein is able to unfold for transmembrane transport.
authors.affiliation: UMR 5539 CNRS, Département Biologie-Santé, Université Montpellier II, France. beaumel@univ-montp2.fr
authors.forename: B
authors.initials: B
authors.lastname: Beaumelle
authors.forename: M P
authors.initials: MP
authors.lastname: Taupiac
authors.forename: J M
authors.initials: JM
authors.lastname: Lord
authors.forename: L M
authors.initials: LM
authors.lastname: Roberts
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D002460:Cell Line; D002474:Cell-Free System; D003600:Cytosol / Q000378:metabolism*; D004705:Endocytosis; D051379:Mice; D017510:Protein Folding; D012276:Ricin / Q000378:metabolism*; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism*
chemical_list: D012276:Ricin; D013762:Tetrahydrofolate Dehydrogenase