The use of the endoplasmic reticulum as a protein storage compartment in cereals: what is needed to make a protein body?

The storage proteins that accumulate in the seeds of cereals and legumes are the major food proteins of the planet. The 2S albumins and the 7S/11S globulins are the most widespread and ancient storage proteins, present in variable amounts in all seeds, and are the major proteins in oilseeds and legumes, respectively. Prolamins, which are evolutionarily derived from the other storage proteins, are instead present only in cereals, where they constitute the major protein fraction. Remarkably, prolamins often form large insoluble polymers, termed protein bodies, which accumulate within the lumen of the endoplasmic reticulum (ER). Conversely the other storage proteins are soluble and traffic along the secretory pathway from the ER to storage vacuoles, where they accumulate. The physiological use of the ER to store proteins seems to be unique to cereals among all eukaryotes, and mutations that negatively affect cereal seed quality often perturb protein body formation. Using maize gamma zeins as a model system, data will be presented on the structural features and molecular interactions that may have determined the change in subcellular localization from vacuoles to the ER without compromising the normal functions of this compartment.