Transport and deposition of cereal prolamins : Targeting and glycosylation of plant secretory proteins

The alcohol-soluble storage proteins of cereal grains, called prolamins, are synthesised on rough endoplasmic reticulum (ER) in the developing starchy endosperm cells and deposited in discrete protein bodies. Two pathways of protein body formation have been proposed : transport via the Golgi apparatus to the vacuole, or direct accumulation within the ER. We discuss and evaluate the evidence for these two pathways, including results of immunoelectron microscopy, characterization of isolated organelles and expression of wild type and mutant proteins in heterologous systems (yeast, Xenopus oocytes and transgenic tobacco). The conclusion is that both pathways may operate, either in different species or in the same tissue at the same or different stages of development. We also discuss the mechanisms which may determine the sorting and trafficking of prolamins within the secretory system, including the roles of ER retention and vacuolar targeting sequences and of molecular chaperones of the HSP70/BiP family, and the organisation of prolamins within the protein body.