title: Structural and biochemical studies identify tobacco SABP2 as a methyl salicylate esterase and implicate it in plant innate immunity.
abstract: Salicylic acid (SA) is a critical signal for the activation of plant defense responses against pathogen infections. We recently identified SA-binding protein 2 (SABP2) from tobacco as a protein that displays high affinity for SA and plays a crucial role in the activation of systemic acquired resistance to plant pathogens. Here we report the crystal structures of SABP2, alone and in complex with SA at up to 2.1-A resolution. The structures confirm that SABP2 is a member of the alpha/beta hydrolase superfamily of enzymes, with Ser-81, His-238, and Asp-210 as the catalytic triad. SA is bound in the active site and is completely shielded from the solvent, consistent with the high affinity of this compound for SABP2. Our biochemical studies reveal that SABP2 has strong esterase activity with methyl salicylate as the substrate, and that SA is a potent product inhibitor of this catalysis. Modeling of SABP2 with MeSA in the active site is consistent with all these biochemical observations. Our results suggest that SABP2 may be required to convert MeSA to SA as part of the signal transduction pathways that activate systemic acquired resistance and perhaps local defense responses as well.
authors.affiliation: Department of Biological Sciences, Northeast Structural Genomics Consortium, Columbia University, New York, NY 10027, USA.
authors.forename: Farhad
authors.initials: F
authors.lastname: Forouhar
authors.forename: Yue
authors.initials: Y
authors.lastname: Yang
authors.forename: Dhirendra
authors.initials: D
authors.lastname: Kumar
authors.forename: Yang
authors.initials: Y
authors.lastname: Chen
authors.forename: Eyal
authors.initials: E
authors.lastname: Fridman
authors.forename: Sang Wook
authors.initials: SW
authors.lastname: Park
authors.forename: Yiwen
authors.initials: Y
authors.lastname: Chiang
authors.forename: Thomas B
authors.initials: TB
authors.lastname: Acton
authors.forename: Gaetano T
authors.initials: GT
authors.lastname: Montelione
authors.forename: Eran
authors.initials: E
authors.lastname: Pichersky
authors.forename: Daniel F
authors.initials: DF
authors.lastname: Klessig
authors.forename: Liang
authors.initials: L
authors.lastname: Tong
mesh_terms: D000595:Amino Acid Sequence; D002256:Carbonic Anhydrases / Q000378:metabolism; D002374:Catalase / Q000378:metabolism; D018360:Crystallography, X-Ray / Q000379:methods; D004950:Esterases; D007113:Immunity, Innate; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism*; D017433:Protein Structure, Secondary; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014026:Nicotiana / Q000201:enzymology*
chemical_list: D010940:Plant Proteins; D002374:Catalase; C499506:SABP2 protein, Nicotiana tabacum; D004950:Esterases; D002256:Carbonic Anhydrases