A GLUTATHIONE-S-TRANSFERASE INVOLVED IN VACUOLAR TRANSFER ENCODED BY THE MAIZE GENE BRONZE-2

GLUTATHIONE S-transferases (GSTs) are enzymes that detoxify heterocyclic compounds (xenobiotics) by covalently linking glutathione to the substrate, forming a glutathione S-conjugate(1,2). A glutathione pump in the vacuolar membrane of barley actively sequesters herbicide-glutathione S-conjugates; glutathionation allows recognition and entry of the conjugates into vacuoles(3). The protein encoded by the Bronze-2 gene in maize performs the last genetically defined step in anthocyanin biosynthesis, resulting in the deposition of red and purple pigments in the vacuoles of maize tissues(4). We show here that Bz2 encodes a GST with activity in maize, transformed Arabidopsis thaliana plants and Escherichia coli. We demonstrate that anthocyanins extracted from maize protoplasts expressing BZ2 are conjugated with glutathione, and that vanadate, a known inhibitor of the glutathione pump(3) in plant vacuolar membranes, inhibits the accumulation of anthocyanins in the vacuole. These results provide a biochemical function for BZ2, and suggest a common mechanism for the ability of plants to sequester structurally similar but functionally diverse molecules in the vacuole.