Felix Stehle, Franziska Götsch, Victor Wray, Jürgen Schmidt, Dieter Strack and Wolfgang Brandt
In this work, it is shown that the At2g23010 gene product encodes 1-O-sinapoyl-β-glucose:1-O-sinapoyl-β-glucose sinapoyltransferase (SST). In contrast to all other functional characterized acyltransferases, the SST protein is highly specific towards this reaction only, and the substrate specificity was correlated to one amino acid substitution. Detailed sequence analysis revealed the lack of the disulphide bond S1 (C78 and C323 in the SMT (sinapoylglucose:malate sinapoyltransferase), that is in SST C80 and D327). The reconstitution of this disulphide bond led to an enzyme accepting many different substrates including disaccharides. Interestingly, the overall changes within the model structures are not very dramatic, but nevertheless, the enzyme models provide some explanations for the broadened substrate specificity: the reconstitution of the disulphide bond provoked more space within the substrate binding pocket simultaneously avoiding electrostatic repulsion. As the SST sequence of A. lyrata also showed the same mutation, the loss of the disulphide bond should has arisen at least 10 mya ago. A Ka/Ks ratio ≤ 1 supports the hypothesis that the loss of this disulphide bond was rather a specification towards a certain reaction than the beginning of a gene death. At the same time, this is also associated with the fixation in the genome.
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