Mechanisms for the Transport of Soluble Proteins to the Vacuole in Plants

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In eukaryotic cells, most proteins destined for the cell surface or for organelles of the secretory pathway are synthesized by endoplasmic-reticulum-bound ribosomes and co-translationally transported into the ER lumen. This translocation process is dependent on the presence of a signal sequence found at the N-terminus of most soluble secretory proteins. If no other signal is present, soluble proteins are transported to the cell surface by a default mechanism, with a positive sorting signal being required for retention within the endomembrane system (reviewed in Bednarek and Raikhel, 1992) [Fig. 1]. Soluble vacuolar proteins (in plant and yeast cells) or lysosomal proteins (in mammalian cells) are transported through the secretory pathway to the trans-Golgi network (TGN), where they are segregated from secreted proteins and carried to the vacuole or lysosome in clathrin-coated vesicles. The mechanisms for sorting proteins to these organelles differ between organisms. Sorting to mammalian lysosomes is usually mediated by oligosaccharide modifications, whereas sorting to yeast and plant vacuoles is mediated by short peptide domains that may be removed before or upon deposition in the organelle, or it may form a part of the mature protein (for review see, Bar-Peled et al., 1996). Soluble proteins are transported through the secretory system via transport vesicles that bud from one compartment and fuse specifically with the next. The specificity of these fusion events is thought to be determined by integral membrane proteins on the vesicles and the acceptor membrane and several candidates for these targeting molecules have recently been identified (Rothman, 1994).