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The major seed storage proteins of maize (Zea mays) and bean (Phaseolus vulgar is), zein and phaseolin, accumulate in the endoplasmic reticulum (ER) and in storage vacuoles, respectively. We show here that a chimeric protein composed of phaseolin and 89 amino acids of γ-zein, including the repeated and the Pro-rich domains, maintains the main characteristics of wild-type γ-zein: It is insoluble unless its disulfide bonds are reduced and forms ER-located protein bodies. Unlike wild-type phaseolin, the protein, which we called zeolin, accumulates to very high amounts in leaves of transgenic tobacco (Nicotiana tabacum). A relevant proportion of the ER chaperone BiP is associated with zeolin protein bodies in an ATP-sensitive fashion. Pulse-chase labeling confirms the high affinity of BiP to insoluble zeolin but indicates that, unlike structurally defective proteins that also extensively interact with BiP, zeolin is highly stable. We conclude that the γ-zein portion is sufficient to induce the formation of protein bodies also when fused to another protein. Because the storage proteins of cereals and legumes nutritionally complement each other, zeolin can be used as a starting point to produce nutritionally balanced and highly stable chimeric storage proteins.
Seeds accumulate very large amounts of a few classes of storage proteins that are used during early germination as a source of reduced nitrogen. Seed storage proteins of legumes and cereals are also the major food proteins for humans, and, nutritionally, they largely complement each other: Storage proteins from legumes are poor in sulfur amino acids and those of cereal are poor in Lys and Trp. Experiments that used genes encoding wild-type or mutated storage proteins to improve by genetic engineering the nutritional value of seeds or vegetative plant tissues indicated that the subcellular location of these proteins is important for stable accumulation (for review, see Tabe and Higgins, 1998). All storage proteins are translocated cotranslationally into the lumen of the endoplasmic reticulum (ER), and then they either accumulate therein or traffic through the secretory pathway to storage vacuoles (for review, see Müntz, 1998). Storage proteins of the 7S and US classes, which are particularly abundant in legumes, accumulate in protein storage vacuoles, whereas cereal prolamins are mainly located in the ER (Müntz, 1998). These proteins are structurally well characterized, and the vacuolar...