Protein ubiquitination is a dynamic and reversible posttranslational modification that controls diverse cellular processes in eukaryotes. Ubiquitin-dependent internalization, recycling, and degradation are important mechanisms that regulate the activity and the abundance of plasma membrane (PM)-localized proteins. In plants, although several ubiquitin ligases are implicated in these processes, no deubiquitinating enzymes (DUBs), have been identified that directly remove ubiquitin from membrane proteins and limit their vacuolar degradation. Here, we discover two DUB proteins, UBP12 and UBP13, that directly target the PM-localized brassinosteroid (BR) receptor BR INSENSITIVE1 (BRI1) in Arabidopsis. BRI1 protein abundance is decreased in the ubp12i/ubp13 double mutant that displayed severe growth defects and reduced sensitivity to BRs. UBP13 directly interacts with and effectively removes K63-linked polyubiquitin chains from BRI1, thereby negatively modulating its vacuolar targeting and degradation. Our study reveals that UBP12 and UBP13 play crucial roles in governing BRI1 abundance and BR signaling activity to regulate plant growth.
Brassinosteroids (BRs) are steroidal phytohormones that are essential for plant growth, development, and adaptation to environmental stresses. BRs act in a dose-dependent manner and they do not travel over long distances, hence, BR homeostasis maintenance is critical for their function. Biosynthesis of bioactive BRs relies on cell-to-cell movement of hormone precursors. However, the mechanism of the short-distance BR transport is unknown and its contribution to control of endogenous BR levels remains unexplored. Here, we demonstrate that plasmodesmata (PD) mediate the passage of BRs between neighboring cells. Intracellular BRs content, in turn, is capable of modulating PD permeability to optimize its own mobility, thereby manipulating BR biosynthesis and signaling. Our work uncovers a thus far unknown mode of steroid transport in eukaryotes and exposes an additional layer of BR homeostasis regulation in plants.
As major components of the ubiquitin system, ubiquitin ligases mediate the transfer of ubiquitin to specific target substrates, thereby playing important roles in regulating a wide range of cellular processes. The Arabidopsis Tóxicos en Levadura (ATL) family is a group of plant-specific RING-type ubiquitin ligases with N-terminal transmembrane-like domains. To date, 91 ATL isoforms have been identified in the Arabidopsis genome, with some reported to regulate plant responses to environmental stresses. However, the functions of most ATLs remain unclear. This study showed that ATL8 is a sugar starvation response gene and that ATL8 expression was significantly increased by sugar starvation conditions but repressed by exogenous sugar supply. The ATL8 protein was found to possess ubiquitin ligase activity in vitro and to localize to membrane-bound compartments in plant cells. In addition, Starch Synthase 4 was identified as a putative interactor with ATL8, suggesting that ATL8 may be involved in modulating starch accumulation in response to sugar availability. These findings suggest that ATL8 functions as a membrane-localized ubiquitin ligase likely to be involved in the adaptation of Arabidopsis plants to sugar starvation stress.
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