Indole-3-acetic acid UDP-glucosyltransferase from immature seeds of pea is involved in modification of glycoproteins.

The glycosylation of auxin is one of mechanisms contributing to hormonal homeostasis. The enzyme UDPG: indole-3-ylacetyl-β-D-glucosyltransferase (IAA glucosyltransferase, IAGlc synthase) catalyzes the reversible reaction: IAA+UDPG↔1-O-IA-glucose+UDP, which is the first step in the biosynthesis of IAA-ester conjugates in monocotyledonous plants. In this study, we report IAA-glucosyltransferase isolated using a biochemical approach from immature seed of pea (Pisum sativum). The enzyme was purified by PEG fractionation, DEAE-Sephacel anion-exchange chromatography and preparative PAGE. LC-MS/MS analysis of tryptic peptides of the enzyme revealed the high identity with maize IAGlc synthase, but lack of homology with other IAA-glucosyltransferases from dicots. Biochemical characterization showed that of several acyl acceptors tested, the enzyme had the highest activity on IAA as the glucosyl acceptor (Km=0.52 mM, Vmax=161 nmol min(-1), kcat/Km=4.36 mM s(-1)) and lower activity on indole-3-propionic acid and 1-naphthalene acetic acid. Whereas indole-3-butyric acid and indole-3-propionic acid were competitive inhibitors of IAGlc synthase, D-gluconic acid lactone, an inhibitor of β-glucosidase activity, potentiated the enzyme activity at the optimal concentration of 0.3mM. Moreover, we demonstrated that the 1-O-IA-glucose synthesized by IAGlc synthase is the substrate for IAA labeling of glycoproteins from pea seeds indicating a possible role of this enzyme in the covalent modification of a class of proteins by a plant hormone.