Receptor proteins mediate the specific cargoes degraded by autophagy. Previously, our work uncovered a potent autophagy receptor for aggregates (2014-Cell) and uncovered its oligomerization feature (2017-Nat Cell Biol.). Further, we found that ribophagy is essential for spermiogenesis (2021-Dev Cell). However, the receptor that mediates ribophagy in spermiogenesis is unknown.
First, we analyzed the controversy that whether or not ribophagy exist. Around 20 different types of cells are analyzed to detect the levels of ribophagy (ribosome proteins) and general autophagy (p62 and LC3) after Tor inhibitor treatment or amino acid starvation. We found that although general autophagy is greatly induced in all these types of cells, ribophagy is induced only in several certain types of cells including male germ cell line. The previously found mammalian ribophagy receptor NUFIP1 is important for the autophagic degradation of ribosomes in male germ cell line. By specific knockout of NUFIP1 in mouse spermatogenic cells, the physiological importance of NUFIP1 as a ribophagy receptor in spermiogenesis is confirmed.
By bioinformatic analysis of mammalian NUFIP1 homolog in yeast, we found Rsa1 as the potential ribophagy receptor in simple eukaryotic cells. Deletion of Rsa1 completely blocked ribophagy while has little effect on general autophagy. Rsa1 binds both the ribosome proteins and Atg8. Tor inhibition or starvation reduces the phosphorylation in the N-terminal regions of Rsa1 and promotes its cytosolic interaction with Atg8 and functions as a ribophagy receptor.
In conclusion, our work uncovered the conservancy and spermiogenesis function of ribophagy receptor NUFIP1/Rsa1.