Vesicle transfer of storage proteins to the vacuole:: The role of the Golgi apparatus and multivesicular bodies

The storage proteins vicilin and legumin pass through the Golgi apparatus (GApp) on their way to the protein storage vacuole (PSV). Upon entering the GApp vicilin and legumin are efficiently segregated to the periphery of the cisternae where they begin to assemble into electron-dense aggregates. These form the nuclei of so-called dense vesicles (DV), which, presumably by cisternal progression through the stack, eventually reach the trans face of the GApp. By this time their contents have become even more condensed and many have obtained a partial clathrin coat on their cytosolic surface. Clathrin coated vesicles (CCV), which appear to bud from the DV may represent a mechanism for the retrieval of proteins missorted into the DV. The DV do not transfer their contents direct to the PSV. Instead, as is the case in yeasts and mammalian cells, an intermediate compartment is implicated. In pea cotyledon cells this takes the form of large (at least 1 mu m diameter), multivesicular bodies (MVB). Double immunogold labeling with globulin antisera suggests that the MVB fuse with the PSV leading to a stratification of storage proteins in deposits at the tonoplast. Experiments with monensin lead to a depletion of the contents of the MVB and a redirection of globulin deposition to the cell surface. CCV, DV and MVB have been probed with antibodies against the vacuolar sorting receptor BP-80. CCV and MVB react positively, whereas there is no signal with the DV.