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-Å resolution. The structures confirm that SABP2 is a member of the ␣͞␤ 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.salicylic acid ͉ salicylic-acid-binding protein ͉ systemic acquired resistance ͉ ␣͞␤ hydrolase

A Role for Differential Glycoconjugation in the Emission of Phenylpropanoid Volatiles from Tomato Fruit Discovered Using a Metabolic Data Fusion Approach

“…These authors demonstrated that MeSA could be transmitted to neighboring plants and subsequently induce their defensive mechanisms by converting MeSA into salicylic acid. The enzyme SABP2 (for salicylic acid-binding protein) mediating the conversion of MeSA into salicylic acid has been identified (Forouhar et al, 2005). Thus, MeSA acts as mobile signal molecule in systemic acquired resistance (Park et al, 2007).…”

Methyl Salicylate Production and Jasmonate Signaling Are Not Essential for Systemic Acquired Resistance inArabidopsis

“…A model has been proposed in which the SA accumulating after tobacco mosaic virus (TMV) infection is converted to MeSA by SA methyl transferase (SAMT1) in inoculated tobacco leaves, and MeSA subsequently travels through the phloem to distant leaves. Here, by the methyl esterase activity of SA binding protein2 (Forouhar et al, 2005), MeSA is reconverted to active SA, which in turn triggers SAR in systemic tissue (Park et al, 2007). In addition to its movement through the phloem, MeSA has been suggested to act as a volatile intraplant signal that is capable of activating SAR in distant leaves of the same plant (Shulaev et al, 1997).…”

Salicylic Acid-Dependent Plant Stress Signaling via Mitochondrial Succinate Dehydrogenase

“…Accumulation of SA is often correlated with an increase in ROS production during plant stress response (for review, see Herrera-Vásquez et al, 2015). A series of SA binding proteins has been identified, notably catalase (Chen et al, 1993a), peroxidase (Durner and Klessig, 1995), and methyl-salicylate esterase (Forouhar et al, 2005) that appear to explain this correlation, but their roles as general SA receptors have been controversial (Attaran et al, 2009;Bi et al, 1995). Further sets of SA binding proteins in Arabidopsis have been identified by affinity screens and include several mitochondrial enzymes and also GSTs including GSTF8, which showed enzymatic inhibition by SA (Manohar et al, 2015;Tian et al, 2012).…”