Wheat storage proteins as a model system to study the mechanism of protein sorting within the endoplasmic reticulum
Although cellular factors that control the sorting of secretory proteins within the endoplasmic reticulum have been identified, very little is known about the signals and mechanisms by which secretory proteins interact with these factors. Wheat storage proteins can serve as an excellent model system for identification of such signals. These proteins are synthesized in developing wheat grains and then accumulate in protein bodies where they survive the period of grain dessication and later serve as a source of nitrogen and energy for the germinating seedlings. The wheat storage proteins are synthesized on membrane bound polysomes and then sequester into the endoplasmic reticulum (ER). Following sequestration, these proteins are either being retained within the ER and form dense protein bodies inside this organelle or are transported via the Golgi to vacuoles and condense into protein bodies at a post-ER location. The signals on these proteins that determine whether they will be retained within the ER or be exported to the Golgi are not known, although they are not of the K/HDEL type as these proteins lack such signals.