To acquire information on the relationships between structural maturation of proteins in the endoplasmic reticulum (ER) and their transport along the secretory pathway, we have analyzed the destiny of an assembly-defective form of the trimeric vacuolar storage glycoprotein phaseolin. In leaves of transgenic tobacco, where assembly-competent phaseolin is correctly targeted to the vacuole, defective phaseolin remains located in the ER or a closely related compartment where it represents a major ligand of the chaperone BiP. Defective phaseolin maintained susceptibility to endoglycosidase H and was slowly degraded by a process that is not inhibited by heat shock or brefeldin A, indicating that degradation does not involve transport along the secretory pathway. These results provide evidence for the presence of a quality control mechanism in the ER of plant cells that avoids intracellular trafficking of severely defective proteins and eventually leads to their degradation.

Phaseolin, one of the major legume proteins for human nutrition, is a trimeric glycoprotein of the 7S class that accumulates in the protein storage vacuoles of common bean. Phaseolin is cotranslationally introduced into the lumen of the endoplasmic reticulum; from there, it is transported through the Golgi complex to the storage vacuoles. Phaseolin is also transported to the vacuole in vegetative tissues of transgenic plants. By transient and permanent expression in tobacco leaf cells, we show here that vacuolar sorting of phaseolin is saturable and that saturation leads to Golgi-mediated secretion from the cell. A mutated phaseolin, in which the four C-terminal residues (Ala, Phe, Val, and Tyr) were deleted, efficiently formed trimers but was secreted entirely outside of the cells in transgenic tobacco leaves, indicating that the deleted sequence contains information necessary for interactions with the saturable vacuolar sorting machinery. In the apoplast, the secreted phaseolin remained intact; this is similar to what occurs to wild-type phaseolin in bean storage vacuoles, whereas in vegetative vacuoles of transgenic plants, the storage protein is fragmented.