Abstract:
Protein abundance and localization at the plasma membrane (PM) shapes plant development and mediates
adaptation to changing environmental conditions. It is regulated by ubiquitination, a post-translational
modification crucial for the proper sorting of endocytosed PM proteins to the vacuole for subsequent
degradation. To understand the significance and the variety of roles played by this reversible modification,
the function of ubiquitin receptors, which translate the ubiquitin signature into a cellular response, needs to
be elucidated. In this study, we show that TOL (TOM1-like) proteins function in plants as multivalent ubiq-
uitin receptors, governing ubiquitinated cargo delivery to the vacuole via the conserved Endosomal Sorting
Complex Required for Transport (ESCRT) pathway. TOL2 and TOL6 interact with components of the ESCRT
machinery and bind to K63-linked ubiquitin via two tandemly arranged conserved ubiquitin-binding do-
mains. Mutation of these domains results not only in a loss of ubiquitin binding but also altered localization,
abolishing TOL6 ubiquitin receptor activity. Function and localization of TOL6 is itself regulated by ubiqui-
tination, whereby TOL6 ubiquitination potentially modulates degradation of PM-localized cargoes, assist-
ing in the fine-tuning of the delicate interplay between protein recycling and downregulation. Taken
together, our findings demonstrate the function and regulation of a ubiquitin receptor that mediates vacu-
olar degradation of PM proteins in higher plants.
