Center for Quantitative Cell Imaging, University of Wisconsin-Madison, Madison, United States; Department of Botany, University of Wisconsin-Madison, Madison, United States
Center for Quantitative Cell Imaging, University of Wisconsin-Madison, Madison, United States
Ariadna Gonzalez Solís
Center for Quantitative Cell Imaging, University of Wisconsin-Madison, Madison, United States; Department of Botany, University of Wisconsin-Madison, Madison, United States
Kuo-En Chen
Department of Biology, Washington University in St. Louis, Saint Louis, United States
Jessica AS Barros
Department of Biology, Washington University in St. Louis, Saint Louis, United States
Santiago Signorelli
ARC Centre of Excellence in Plant Energy Biology, School of Molecular Sciences, The University of Western Australia, Perth, Australia; Department of Plant Biology,School of Agronomy, Universidad de la República, Montevideo, Uruguay
A Harvey Millar
ARC Centre of Excellence in Plant Energy Biology, School of Molecular Sciences, The University of Western Australia, Perth, Australia
Center for Quantitative Cell Imaging, University of Wisconsin-Madison, Madison, United States; Department of Medical Physics, University of Wisconsin-Madison, Madison, United States; Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, United States; Morgridge Institute for Research, Madison, United States
Center for Quantitative Cell Imaging, University of Wisconsin-Madison, Madison, United States; Department of Botany, University of Wisconsin-Madison, Madison, United States
The ubiquitin-binding NBR1 autophagy receptor plays a prominent role in recognizing ubiquitylated protein aggregates for vacuolar degradation by macroautophagy. Here, we show that upon exposing Arabidopsis plants to intense light, NBR1 associates with photodamaged 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 translocon 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.
