The endoplasmic reticulum as a protein storage compartment

In all eukaryotic cells, the endoplasmic reticulum (ER) is the protein nursery of the secretory pathway. Folding helpers present in the ER monitor and help the productive folding of newly synthesized proteins destined to vacuoles (lysosomes in animals), the cell surface, or other intermediate conpartments of the endomembrane system, and promote the degradation of polypeptides that fail to reach correct folding. Like any nursery, the ER is therefore mainly a compartment of transit of the newborn, rather than a permanent residence. Consistently, high protein accumulation in the ER is associated to many human diseases. However, plants also use the ER to store proteins. The major and more spectacular example is constituted by the seed storage proteins that accumulate in the endosperm cells of cereal seeds. These proteins are unique to plants and are of great agricultural importance, being the main protein source for human nutrition. Using maize storage proteins as a model system, this lecture will illustrate recent advancements in our knowledge of the protein structural features and molecular interactions that allow high accumulation of protein in the plant ER without compromising the constitutive functions of this subcellular compartment. Supported by the FILAGRO Project of CNR-Regione Lombardia.