A glutathione S-transferase involved in vacuolar transfer encoded by the maize gene Bronze-2

Abstract

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 vacuoles³. 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 tissues⁴. 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 pump³ 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.