PROTEIN-QUALITY CONTROL ALONG THE ROUTE TO THE PLANT VACUOLE

To acquire information on the relationships between structural maturat ion of proteins in the endoplasmic reticulum (ER) and their transport along the secretory pathway, we have analyzed the destiny of an assemb ly-defective form of the trimeric vacuolar storage glycoprotein phaseo lin. In leaves of transgenic tobacco, where assembly-competent phaseol in is correctly targeted to the vacuole, defective phaseolin remains l ocated in the ER or a closely related compartment where it represents a major ligand of the chaperone Dip. Defective phaseolin maintained su sceptibility to endoglycosidase H and was slowly degraded by a process that is not inhibited by heat shock or brefeldin A, indicating that d egradation does not involve transport along the secretory pathway. The se results provide evidence for the presence of a quality control mech anism in the ER of plant cells that avoids intracellular trafficking o f severely defective proteins and eventually leads to their degradatio n.