The autophagy receptor NBR1 directs the clearance of photodamaged chloroplasts

Han Nim Lee, Jenu Varghese Chacko, Ariadna Gonzalez Solís, Kuo En Chen, Jessica A.S. Barros, Santiago Signorelli, A. Harvey Millar, Richard David Vierstra, Kevin W. Eliceiri, Marisa S. Otegui

Research output: Contribution to journalArticlepeer-review

5 Scopus citations

Abstract

The ubiquitin-binding NBR1 autophagy receptor plays a prominent role in recog-nizing ubiquitylated protein aggregates for vacuolar degradation by macroautophagy. Here, we show that upon exposing Arabidopsis plants to intense light, NBR1 associates with photodam-aged chloroplasts independently of ATG7, a core component of the canonical autophagy machinery. NBR1 coats both the surface and interior of chloroplasts, which is then followed by direct engulfment of the organelles into the central vacuole via a microautophagy-type process. The relocalization of NBR1 into chloroplasts does not require the chloroplast trans-locon complexes embedded in the envelope but is instead greatly enhanced by removing the self-oligomerization mPB1 domain of NBR1. The delivery of NBR1-decorated chloroplasts into vacuoles depends on the ubiquitin-binding UBA2 domain of NBR1 but is independent of the ubiquitin E3 ligases SP1 and PUB4, known to direct the ubiquitylation of chloroplast surface proteins. Compared to wild-type plants, nbr1 mutants have altered levels of a subset of chloroplast proteins and display abnormal chloroplast density and sizes upon high light exposure. We postulate that, as photodamaged chloroplasts lose envelope integrity, cytosolic ligases reach the chloroplast interior to ubiquitylate thylakoid and stroma proteins which are then recognized by NBR1 for autophagic clearance. This study uncovers a new function of NBR1 in the degradation of damaged chloroplasts by microautophagy.

Original languageEnglish
Article numbere86030
JournaleLife
Volume12
DOIs
StatePublished - Apr 2023

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