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Plant Hormone Modulators
The activity and stability of several plant hormones is modulated by conjugation with various amino acids and their derivatives. Westfall et al. (p. 1708, published online 24 May) solved the crystal structures for two acyl acid amido synthetases from Arabidopsis. The findings suggest how the enzymes might discriminate between apolar and acidic amino acids and lend insight into the reaction chemistries that add functional diversity to hormone signaling pathways.
Acyl acid amido synthetases of the GH3 family act as critical prereceptor modulators of plant hormone action; however, the molecular basis for their hormone selectivity is unclear. Here, we report the crystal structures of benzoate-specific Arabidopsis thaliana AtGH3.12/PBS3 and jasmonic acid–specific AtGH3.11/JAR1. These structures, combined with biochemical analysis, define features for the conjugation of amino acids to diverse acyl acid substrates and highlight the importance of conformational changes in the carboxyl-terminal domain for catalysis. We also identify residues forming the acyl acid binding site across the GH3 family and residues critical for amino acid recognition. Our results demonstrate how a highly adaptable three-dimensional scaffold is used for the evolution of promiscuous activity across an enzyme family for modulation of plant signaling molecules.