Overexpression of a gene that encodes the first enzyme in the biosynthesisof asparagine-linked glycans makes plants resistant to tunicamycin and obviates the tunicamycin-induced unfolded protein response

The cytotoxic drug tunicamycin kills cells because it is a specific inhibit or of UDP-N-acetylglucosamine:dolichol phosphate N-acetylglucosamine-1-P tr ansferase (GPT), an enzyme that catalyzes the initial step of the biosynthe sis of dolichol-linked oligosaccharides. In the presence of tunicamycin, as paragine-linked glycoproteins made in the endoplasmic reticulum are not gly cosylated with N-linked glycans, and therefore may not fold correctly. Such proteins may be targeted for breakdown. Cells that are treated with tunica mycin normally experience an unfolded protein response and induce genes tha t encode endoplasmic reticulum chaperones such as the binding protein (BiP) . We isolated a cDNA clone for Arabidopsis GPT and overexpressed it in Arab idopsis. The transgenic plants have a 10-fold higher level of GPT activity and are resistant to 1 mu g/mL tunicamycin, a concentration that kills cont rol plants. Transgenic plants grown in the presence of tunicamycin have N-g lycosylated proteins and the drug does not induce BiP mRNA levels as it doe s in control plants. BiP mRNA levels are highly induced in both control and CPT-expressing plants by azetidine-2-carboxylate. These observations sugge st that excess GPT activity obviates the normal unfolded protein response t hat cells experience when exposed to tunicamycin.