title: A plant pathogen virulence factor inhibits the eukaryotic proteasome by a novel mechanism.
abstract: Pathogenic bacteria often use effector molecules to increase virulence. In most cases, the mode of action of effectors remains unknown. Strains of Pseudomonas syringae pv. syringae (Pss) secrete syringolin A (SylA), a product of a mixed non-ribosomal peptide/polyketide synthetase, in planta. Here we identify SylA as a virulence factor because a SylA-negative mutant in Pss strain B728a obtained by gene disruption was markedly less virulent on its host, Phaseolus vulgaris (bean). We show that SylA irreversibly inhibits all three catalytic activities of eukaryotic proteasomes, thus adding proteasome inhibition to the repertoire of modes of action of virulence factors. The crystal structure of the yeast proteasome in complex with SylA revealed a novel mechanism of covalent binding to the catalytic subunits. Thus, SylA defines a new class of proteasome inhibitors that includes glidobactin A (GlbA), a structurally related compound from an unknown species of the order Burkholderiales, for which we demonstrate a similar proteasome inhibition mechanism. As proteasome inhibitors are a promising class of anti-tumour agents, the discovery of a novel family of inhibitory natural products, which we refer to as syrbactins, may also have implications for the development of anti-cancer drugs. Homologues of SylA and GlbA synthetase genes are found in some other pathogenic bacteria, including the human pathogen Burkholderia pseudomallei, the causative agent of melioidosis. It is thus possible that these bacteria are capable of producing proteasome inhibitors of the syrbactin class.
authors.affiliation: Center for Integrated Protein Science at the Department Chemie, Lehrstuhl für Biochemie, Technische Universität München, Lichtenbergstrasse 4, Garching D-85747, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Groll
authors.forename: Barbara
authors.initials: B
authors.lastname: Schellenberg
authors.forename: André S
authors.initials: AS
authors.lastname: Bachmann
authors.forename: Crystal R
authors.initials: CR
authors.lastname: Archer
authors.forename: Robert
authors.initials: R
authors.lastname: Huber
authors.forename: Tracy K
authors.initials: TK
authors.lastname: Powell
authors.forename: Steven
authors.initials: S
authors.lastname: Lindow
authors.forename: Markus
authors.initials: M
authors.lastname: Kaiser
authors.forename: Robert
authors.initials: R
authors.lastname: Dudler
mesh_terms: D000970:Antineoplastic Agents / Q000145:classification / Q000494:pharmacology; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism; D016957:Burkholderia pseudomallei / Q000235:genetics; D002384:Catalysis / Q000187:drug effects; D003460:Crystallization; D005057:Eukaryotic Cells / Q000201:enzymology*; D006801:Humans; D010206:Papain / Q000378:metabolism; D010456:Peptides, Cyclic / Q000145:classification / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology*; D027805:Phaseolus / Q000201:enzymology / Q000382:microbiology*; D046988:Proteasome Endopeptidase Complex / Q000737:chemistry / Q000378:metabolism; D061988:Proteasome Inhibitors; D021122:Protein Subunits / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism; D044224:Pseudomonas syringae / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D013379:Substrate Specificity; D014357:Trypsin / Q000378:metabolism; D014774:Virulence / Q000235:genetics; D037521:Virulence Factors / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D000970:Antineoplastic Agents; D010456:Peptides, Cyclic; D061988:Proteasome Inhibitors; D021122:Protein Subunits; D037521:Virulence Factors; C407267:syringolin A; C057878:glidobactin A; D014357:Trypsin; D010206:Papain; D046988:Proteasome Endopeptidase Complex