Dual roles of AtNBR1 in regulating selective autophagy <i>via</i> liquid-liquid phase separation and recognition of non-ubiquitinated substrates in Arabidopsis

Selective macroautophagy/autophagy in metazoans involves the conserved receptors NBR1 and SQSTM1/p62. Both autophagy receptors manage ubiquitinated cargo recognition, while SQSTM1 has an additional, distinct role of facilitating liquid-liquid phase separation (LLPS) during autophagy. Given that plants lack SQSTM1, it is postulated that plant NBR1 may combine activities of both metazoan NBR1 and SQSTM1. However, the precise mechanism by which plant NBR1 recognizes non-ubiquitinated substrates and its ability to undergo LLPS during selective autophagy remain elusive. Here, we implicate both the ZZ-type zinc finger motif and the four-tryptophan domain of Arabidopsis NBR1 (AtNBR1) in the recognition of non-ubiquitinated cargo proteins. Additionally, we reveal that AtNBR1 indeed undergoes LLPS prior to ATG8-mediated autophagosome formation, crucial for heat stress resistance in Arabidopsis. Our findings unveil the dual roles of AtNBR1 in both cargo recognition and LLPS during plant autophagy and advance our understanding of NBR1-mediated autophagy in plants compared to metazoans.<b>Abbreviations</b>: ATG8: autophagy 8; Co-IP: co-immunoprecipitation; EXO70E2: exocyst subunit EXO70 family protein E2; FRAP: fluorescence recovery after photobleaching; FW domain: four-tryptophan domain; GFP: green fluorescent protein; HS: heat stress; LLPS: liquid-liquid phase separation; LIR: LC3-interacting region; NBR1: next to BRCA1 gene 1; PAS: phagophore assembly site; PB1 domain: Phox and Bem1 domain; RFP: red fluorescent protein; ROF1: rotamase FKBP 1; SARs: selective autophagy receptors; UBA domain: ubiquitin-associated domain; Y2H: yeast two-hybrid; ZZ domain: ZZ-type zinc finger motif domain.