title: Native and Foreign Proteins Secreted by theCupriavidus metalliduransType II System and an Alternative Mechanism.
abstract: The type II secretion system (T2SS), which transports selected periplasmic proteins across the outer membrane, has rarely been studied in nonpathogens or in organisms classified as Betaproteobacteria. Therefore, we studiedCupriavidus metallidurans(Cme), a facultative chemilithoautotroph. Gel analysis of extracellular proteins revealed no remarkable differences between the wild type and the T2SS mutants. However, enzyme assays revealed that native extracellular alkaline phosphatase is a T2SS substrate, because activity was 10-fold greater for the wild type than a T2SS mutant. InCmeengineered to produce threeRalstonia solanacearum(Rso) exoenzymes, at least 95% of their total activities were extracellular, but unexpectedly high percentages of these exoenzymes remained extracellular in T2SS mutants cultured in rich broth. These conditions appear to permit an alternative secretion process, because neither cell lysis nor periplasmic leakage was observed whenCmeproduced aPectobacterium carotovorumexoenzyme, and wild-typeCmecultured in minimal medium secreted 98% ofRsopolygalacturonase, but 92% of this exoenzyme remained intracellular in T2SS mutants. We concluded thatCmehas a functional T2SS despite lacking any abundant native T2SS substrates. The efficient secretion of three foreign exoenzymes byCmeis remarkable, but so too is the indication of an alternative secretion process in rich culture conditions. When not transiting the T2SS, we suggest thatRsoexoenzymes are probably selectively packaged into outer membrane vesicles. Phylogenetic analysis of T2SS proteins supports the existence of at least three T2SS subfamilies, and we propose thatCme, as a representative of the Betaproteobacteria, could become a new useful model system for studying T2SS substrate specificity.
authors.affiliation: College of Plant Protection, Shandong Agricultural University, Tai'an, Shandong 271018, P.R. China.
authors.forename: Houjuan
authors.initials: H
authors.lastname: Xu
authors.affiliation: Department of Plant Pathology, The University of Georgia, Athens, GA 30602, USA.
authors.forename: Timothy P
authors.initials: TP
authors.lastname: Denny
mesh_terms: D000469:Alkaline Phosphatase / Q000235:genetics / Q000378:metabolism; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport; D002265:Carboxylic Ester Hydrolases / Q000235:genetics / Q000378:metabolism; D002480:Cellulase / Q000235:genetics / Q000378:metabolism; D049921:Cupriavidus / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism*; D004269:DNA, Bacterial; D057075:Enzyme Assays; D004926:Escherichia coli / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D005810:Multigene Family / Q000235:genetics; D009154:Mutation; D016973:Pectobacterium carotovorum / Q000201:enzymology; D010802:Phylogeny; D011096:Polygalacturonase / Q000235:genetics / Q000378:metabolism; D000072417:Protein Domains; D017433:Protein Structure, Secondary; D000069284:Protein Translocation Systems / Q000145:classification / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D043368:Ralstonia solanacearum / Q000201:enzymology; D016415:Sequence Alignment; D000069178:Type II Secretion Systems / Q000145:classification / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D000069284:Protein Translocation Systems; D000069178:Type II Secretion Systems; D002265:Carboxylic Ester Hydrolases; C022323:pectinesterase; D000469:Alkaline Phosphatase; D011096:Polygalacturonase; D002480:Cellulase
keywords: Type II secretion system; alkaline phosphatase; endoglucanase; outer membrane vesicles; pectin methyl esterase; phylogeny; polygalacturonase; protein secretion