Accumulation of 15-kilodalton zein in novel protein bodies in transenic tobacco

Abstract

Zeins, the seed storage proteins of maize, are a group of alcohol-soluble polypeptides of different molecular masses that share a similar amino acid composition but vary in their sulfur amino acid composition. They are synthesized on the rough endoplasmic reticulum (ER) in the endosperm and are stored in ER-derived protein bodies. Our goal is to balance the amino acid composition of the methionine-deficient forage legumes by expressing the sulfur amino acid-rich 15-kD zeins in their leaves. However, it is crucial to know whether this protein would be stable in nonseed tissues of trans-genic plants. The major focus of this paper is to compare the accumulation pattern of the 15-kD zein protein with a vacuolar targeted seed protein, β-phaseolin, in nonseed tissues and to determine the basis for its stability/instability. We have introduced the 15-kD zein and bean β-phaseolin-coding sequences behind the 35S cauliflower mosaic virus promoter into tobacco (Nicotiana tabacum) and analyzed the protein's accumulation pattern in different tissues. Our results demonstrate that the 15-kD seed protein is stable not only in seeds but in all nonseed tissues tested, whereas the β-phaseolin protein accumulated only in mid- and postmaturation seeds. Interestingly, zein accumulates in novel protein bodies both in the seeds and in nonseed tissues. We attribute the instability of the β-phaseolin protein in nonseed tissues to the fact that it is targeted to protease-rich vacuoles. The stability of the 15-kD zein could be attributed to its retention in the ER or to the proteaseresistant nature of the protein.