title: Identification of the ricin lipase site and implication in cytotoxicity.
abstract: Ricin is a heterodimeric plant toxin and the prototype of type II ribosome-inactivating proteins. Its B-chain is a lectin that enables cell binding. After endocytosis, the A-chain translocates through the membrane of intracellular compartments to reach the cytosol where its N-glycosidase activity inactivates ribosomes, thereby arresting protein synthesis. We here show that ricin possesses a functional lipase active site at the interface between the two subunits. It involves residues from both chains. Mutation to alanine of catalytic serine 221 on the A-chain abolished ricin lipase activity. Moreover, this mutation slowed down the A-chain translocation rate and inhibited toxicity by 35%. Lipase activity is therefore required for efficient ricin A-chain translocation and cytotoxicity. This conclusion was further supported by structural examination of type II ribosome-inactivating proteins that showed that this lipase site is present in toxic (ricin and abrin) but is altered in nontoxic (ebulin 1 and mistletoe lectin I) members of this family.
authors.affiliation: UMR 5539 CNRS, Département Biologie-Santé, Université Montpellier II, 34095 Montpellier, France.
authors.forename: Juliette
authors.initials: J
authors.lastname: Morlon-Guyot
authors.forename: Mohamed
authors.initials: M
authors.lastname: Helmy
authors.forename: Sophie
authors.initials: S
authors.lastname: Lombard-Frasca
authors.forename: David
authors.initials: D
authors.lastname: Pignol
authors.forename: Gérard
authors.initials: G
authors.lastname: Piéroni
authors.forename: Bruno
authors.initials: B
authors.lastname: Beaumelle
mesh_terms: D004789:Enzyme Activation; D007700:Kinetics; D008049:Lipase / Q000032:analysis* / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D011487:Protein Conformation; D021122:Protein Subunits; D012276:Ricin / Q000032:analysis* / Q000737:chemistry / Q000378:metabolism
chemical_list: D021122:Protein Subunits; D012276:Ricin; D008049:Lipase