Zeolin, a new recombinant storage protein constructed using maize ×-zein and bean phaseolin

The major seed storage proteins of Zea mais and Phaseolus vulgaris, zein and phaseolin, accumulate in the endoplasmic reticulum and in storage vacuoles, respectively. We show here that a chimeric protein composed of phaseolin and the first 89 amino acids of g-zein, which we called zeolin, maintains the main characteristics of wild-type g-zein: it is insoluble unless its disulfide bonds are reduced and forms endoplasmic reticulum-located protein bodies. Unlike wild-type phaseolin, the protein accumulates to very high amounts in leaves of transgenic tobacco (Nicotiana tabacum). A relevant proportion of the endoplasmic reticulum 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 g-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.