Mecchanism of insoluble protein body formation by the maize storage protein gamma-zein

Maize seed storage proteins (zeins) accumulate as insoluble heteropolymers (protein bodies) in the endoplasmic reticulum (ER). Gamma-zein is a major component, assembles into protein bodies also when synthesized in the absence of its partners and is composed of a C-terminal domain with four intrachain disulfide bonds, preceded by an N-terminal part that contains seven additional Cys residues and a Pro-rich repeated region. By transient expression in tobacco protoplasts, we have analyzed the relationships between assembly, solubility and intracellular traffic of gamma-zein in which either the first two, six or all Cys residues of the N-terminal part have been mutated to Ser. Our results indicate that gamma-zein can assemble as soluble dimers, most probably through hydrophobic interactions between the repeated domains. In the absence of further assembly, these dimers are largely secreted and in part sorted to the vacuole The formation of interchain disulfide bonds, to an extent that depends on the availability of Cys residues in the N-terminal half, allows further assembly into insoluble polymers and inhibits traffic from the ER. Supported by the 2006 Accordo Quadro CNR-Regione Lombardia.