K63-linked ubiquitin chains are a global signal for endocytosis and contribute to selective autophagy in plants

In contrast to other eukaryotic model organisms, the closely related ubiquitin (Ub)-conjugating enzymes UBC35 and UBC36 are the main sources of K63-linked Ub chains in Arabidopsis.1 Although K63-linked chains have been associated with the regulation of vesicle trafficking, definitive proof for their role in endocytosis was missing. We show that the ubc35 ubc36 mutant has pleiotropic phenotypes related to hormone and im-mune signaling. Specifically, we reveal that ubc35-1 ubc36-1 plants have altered turnover of integral mem-brane proteins including FLS2, BRI1, and PIN1 at the plasma membrane. Our data indicates that K63-Ub chains are generally required for endocytic trafficking in plants. In addition, we show that in plants K63-Ub chains are involved in selective autophagy through NBR1, the second major pathway delivering cargoes to the vacuole for degradation. Similar to autophagy-defective mutants, ubc35-1 ubc36-1 plants display an accumulation of autophagy markers. Moreover, autophagy receptor NBR1 interacts with K63-Ub chains, which are required for its delivery to the lytic vacuole.2 Together, we show that K63-Ub chains act as a general signal required for the two main pathways delivering cargo to the vacuole and thus, to maintain proteostasis.

Automated summary

Unlike other eukaryotic model organisms, the main sources of K63-linked Ub chains in Arabidopsis are UBC35 and UBC36. Previous research has associated K63-linked chains with vesicle trafficking regulation but lacked definitive evidence for their role in endocytosis. This study demonstrates that K63-Ub chains are essential for endocytic trafficking in plants and are also involved in selective autophagy through the interaction with NBR1. These findings highlight the importance of K63-Ub chains in maintaining proteostasis in plants.