Organelles and signals controlling vacuolar sorting receptor trafficking in plants

The maintenance of vacuoles/lysosomes is essential for the physiology of all eukaryotic life. In plants, sorting of soluble protein cargo to the lytic vacuoles/lysosomes is controlled by a family of type-I membrane spanning proteins, the Vacuolar Sorting Receptors (VSRs). Whilst the large lumenal domain mediates conditional ligand binding and release, the short cytosolic tail differentiates between anterograde and retrograde transport machinery in the cytosol. In this thesis, experimental tools and techniques are developed and used to characterise the sorting cycle of plants. In contrast to previous studies, work presented here suggests that the different isoforms are not transported in an identical manner. In particular, one VSR member was characterised to have increased leakage to the vacuole, partitioning to the late-prevacuolar compartment rather than the prevacuolar compartment. Both the cytosolic C-terminus and the cargo interacting lumenal domain of this unique receptor were shown to interact differentially with partners when compared to the canonical receptor. In addition, the techniques developed here are used to characterise the sorting route of the canonical receptor. A region of the receptor important for export from the endoplasmic reticulum mediating its entry into COPII vesicles has been identified. Furthermore, this model receptor is also used to understand the general sorting of membrane spanning proteins within the secretory pathway, showing that the presence of a tyrosine motif within the C-terminus prevents default leakage towards the plasma membrane. Finally, it is shown here that the VSR does not traffic via the plasma membrane by default and thus probably traffics in clathrin coated vesicles from the Golgi apparatus/trans-Golgi network towards the late secretory pathway, despite containing a functional endocytic motif.