title: Fusion of dioxygenase and lignin-binding domains in a novel secreted enzyme from cellulolytic Streptomyces sp. SirexAA-E.
abstract: Streptomyces sp. SirexAA-E is a highly cellulolytic bacterium isolated from an insect/microbe symbiotic community. When grown on lignin-containing biomass, it secretes SACTE_2871, an aromatic ring dioxygenase domain fused to a family 5/12 carbohydrate-binding module (CBM 5/12). Here we present structural and catalytic studies of this novel fusion enzyme, thus providing insight into its function. The dioxygenase domain has the core β-sandwich fold typical of this enzyme family but lacks a dimerization domain observed in other intradiol dioxygenases. Consequently, the x-ray structure shows that the enzyme is monomeric and the Fe(III)-containing active site is exposed to solvent in a shallow depression on a planar surface. Purified SACTE_2871 catalyzes the O2-dependent intradiol cleavage of catechyl compounds from lignin biosynthetic pathways, but not their methylated derivatives. Binding studies show that SACTE_2871 binds synthetic lignin polymers and chitin through the interactions of the CBM 5/12 domain, representing a new binding specificity for this fold-family. Based on its unique structural features and functional properties, we propose that SACTE_2871 contributes to the invasive nature of the insect/microbial community by destroying precursors needed by the plant for de novo lignin biosynthesis as part of its natural wounding response.
authors: Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA.
authors: Christopher M
authors: CM
authors: Bianchetti
authors: Connor H
authors: CH
authors: Harmann
authors: Taichi E
authors: TE
authors: Takasuka
authors: Gregory L
authors: GL
authors: Hura
authors: Kevin
authors: K
authors: Dyer
authors: Brian G
authors: BG
authors: Fox
mesh_terms: D001426:Bacterial Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites / Q000235:genetics; D055162:Biocatalysis; D002396:Catechols / Q000737:chemistry / Q000378:metabolism; D002482:Cellulose / Q000378:metabolism; D002686:Chitin / Q000378:metabolism; D018360:Crystallography, X-Ray; D049308:Dioxygenases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D050939:Gene Fusion; D007501:Iron / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D008031:Lignin / Q000378:metabolism*; D008958:Models, Molecular; D015394:Molecular Structure; D010100:Oxygen / Q000378:metabolism; D010802:Phylogeny; D011485:Protein Binding; D055503:Protein Multimerization; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013302:Streptomyces / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D001426:Bacterial Proteins; D002396:Catechols; D002686:Chitin; D002482:Cellulose; D008031:Lignin; D007501:Iron; D049308:Dioxygenases; C034221:catechol; D010100:Oxygen
keywords: Caffeoyl-CoA; Carbohydrate-binding Protein; Dioxygenase; Iron; Lignin; Lignin Degradation; Protein Structure; SAXS
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