Protein Targeting to the Vacuole in Plant Cells

The vacuole in plant cells plays various roles that are important in the maintenance of cell organization and func- tion, yet the vacuolar function varies greatly depending on the type of cell and on the stage of plant development. The regulation of deposition of various proteins with specific functions in the vacuole is a prerequisite for the expression and maintenance of vacuolar functions. Transport of proteins from the site of synthesis to the site of deposition is generally mediated by topogenic information in the passenger protein and the cellular apparatus that recognizes it. Recent identifi- cation of some of the determinants that affect transport and sorting of proteins to the vacuole opened a new pathway to reveal how protein targeting to the vacuole can be regulated. TRAFFICKING AND SORTING OF VACUOLAR PROTEINS The vacuole is one of the organelles that constitute the complex secretory system of the cell, similar to the lysosome of animal cells and the vacuole of yeast. A11 the membranes in the secretory system are connected by traffic of various small transport vesicles, and the lumen of these membrane compartments is topologically identical to the extracellular milieu. Most, if not all, of the proteins in this compartment first enter the lumen of the ER by direction of the signal peptide in the precursor, and then they are transported to appropriate destinations without any further translocation through a membrane. Secretory proteins pass through various compartments of the Golgi apparatus, the TGN, and the secretory vesicles before they are secreted from the cell. Vacuolar proteins follow the same pathway until they are separated from secretory proteins, probably in the TGN, for specific transport to the vacuole. Basically, secretion is a bulk- flow default pathway in plants, and proteins without any topogenic information other than the signal peptide are se- creted (Chrispeels, 199 1). Vacuolar proteins should be sorted from secretory proteins at the branch point of the transport pathway by a sorting machinery that recognizes a sorting signal in the vacuolar protein.