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.