Ubiquitin initiates sorting of Golgi and plasma membrane proteins into the vacuolar degradation pathway

Ubiquitin-tagged PM reporters localized to endosomes and to the lumen of the lytic vacuole in tobacco mesophyll protoplasts and in tobacco epidermal cells. The internalization of these reporters was significantly reduced if clathrin-mediated endocytosis was inhibited by the coexpression of a mutant of the clathrin heavy chain, the clathrin hub.？Surprisingly, a ubiquitin-tagged reporter for the Golgi was also transported into the lumen of the vacuole. Vacuolar delivery of the reporters was abolished upon inhibition of the ESCRT machinery, indicating that the vacuolar delivery of these reporters occurs via the endocytic transport route.Ubiquitin acts as a sorting signal at different compartments in the endomembrane system to target membrane proteins into the vacuolar degradation pathway: If displayed at the PM, ubiquitin triggers internalization of PM reporters into the endocytic transport route, but it also mediates vacuolar delivery if displayed at the Golgi. In both cases, ubiquitin-tagged proteins travel via early endosomes and multivesicular bodies to the lytic vacuole. This suggests that vacuolar degradation of ubiquitinated proteins is not restricted to PM proteins but might also facilitate the turnover of membrane proteins in the early secretory pathway.The endocytic uptake of proteins and lipids is the driving force that establishes and maintains cellular polarity, but also allows for intercellular communication and facilitates interactions with the environment [1,2]. Endocytosis involves invagination and fission of vesicles at the plasma membrane (PM) and their transport to endosomes. Endocytosis in walled plant cells has been shown to exist by the use of fluorescent dyes in the early 2000s and has been confirmed by the subsequent identification of endocytic cargo molecules like the auxin efflux facilitator PINFORMED 1 (PIN1) [3] or cell surface receptors like the brassinosteroid receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) and the flagellin receptor FLAGEL