Update on Cell Biology Endoplasmic Reticulum-Derived Compartments Function in Storage and as Mediators of Vacuolar Remodeling via a New Type of Organelle, Precursor Protease Vesicles

The endoplasmic reticulum (ER) is the gateway to the secretory pathway, and the proteins that are made and assembled in the ER can have a variety of cellular destinations (Vitale and Denecke, 1999). Most proteins proceed to these destinations by progression through the endomembrane system via the Golgi apparatus. This pathway of protein transport has been extensively studied in plant, mammalian, and yeast cells. In most instances proteins move directly to the Golgi within minutes of being synthesized, but some proteins and other molecules can apparently be stored for shorter or longer periods of time in ERderived compartments. In some cases these ERderived compartments travel to and are incorporated into vacuoles. Plant cells appear to have flexibility in using the ER to assemble storage organelles. So far, ER-derived storage organelles such as protein bodies and oil bodies have been described primarily in seeds. Recent results substantiate a long-standing proposal for a role for the ER in vacuolar ontogeny in storage tissues of young seedlings by forming small “second vacuoles” (here called precursor protease vesicles [PPVs]) that are involved in the mobilization of proteins in the protein storage vacuoles (PSVs). A variety of designations have been used to describe ER-derived compartments, and in this Update we attempt to bring together seemingly disparate observations. We show that retention of proteins in the ER gives rise to storage compartments that may continue to exist as such or later merge with or be autophaged by vacuoles. The Update does not deal with the wellcharacterized ER-to-Golgi transport pathway that has been discussed in the context of storage protein deposition in recent reviews (Herman and Larkins, 1999; Vitale and Denecke, 1999).