An acyltransferase catalyzing the formation of diacylglucose is a serine carboxypeptidase-like protein

1-O-beta-acyl acetals serve as activated donors in group transfer reactions involved in plant natural product biosynthesis and hormone metabolism. How ever, the acyltransferases that mediate transacylation from 1-O-beta-acyl a cetals have not been identified. We report the identification of a cDNA enc oding a 1-O-beta-acylglucose-dependent acyltransferase functioning in gluco se polyester biosynthesis by Lycopersicon pennellii, The acyltransferase cD NA encodes a serine carboxypeptidase-like protein, with a conserved Ser-His -Asp catalytic triad, Expression of the acyltransferase cDNA in Saccharomyc es cerevisiae conferred the ability to disproportionate 1-O-beta-acylglucos e to diacylglucose. The disproportionation reaction is regiospecific, catal yzing the conversion of two equivalents of 1-O-beta-acylglucose to 1,2-di-O -acylglucose and glucose, Diisopropyl fluorophosphate, a transition-state a nalog inhibitor of serine carboxypeptidases, inhibited acyltransferase acti vity and covalently labeled the purified acyltransferase, suggesting the in volvement of an active serine in the mechanism of the transacylation, The a cyltransferase exhibits no carboxypeptidase activity; conversely, the serin e carboxypeptidases we have tested show no ability to transacylate using 1- O-acyl-beta-glucoses. This acyltransferase may represent one member of a br oader class of enzymes recruited from proteases that have adapted a common catalytic mechanism of catabolism and modified it to accommodate a wide ran ge of group transfer reactions used in biosynthetic reactions of secondary metabolism. The abundance of serine carboxypeptidase-like proteins in plant s suggests that this motif has been used widely for metabolic functions.