Atg20- and Atg24-family proteins promote organelle autophagy in fission yeast

Abstract

Autophagy cargos include not only soluble cytosolic materials but also bulky organelles, such as ER and mitochondria. In budding yeast, two proteins that contain the PX domain and the BAR domain, Atg20 and Atg24 (also known as Snx42 and Snx4, respectively) are required for organelle autophagy and contribute to general autophagy in a way that can be masked by compensatory mechanisms. It remains unclear why these proteins are important for organelle autophagy. Here, we show that in a distantly related fungal organism, the fission yeast Schizosaccharomyces pombe, autophagy of ER and mitochondria is induced by nitrogen starvation and is promoted by three Atg20-and Atg24-family proteins -Atg20, Atg24 and SPBC1711.11 (named here as Atg24b). These proteins localize at the pre-autophagosomal structure, or phagophore assembly site (PAS), during starvation. S. pombe Atg24 forms a homo-oligomer and acts redundantly with Atg20 and Atg24b, and the latter two proteins can form a hetero-oligomer. The organelle autophagy defect caused by the loss of these proteins is associated with a reduction of autophagosome size and a decrease in Atg8 accumulation at the PAS. These results provide new insights into the autophagic function of Atg20-and Atg24-family proteins.

Autophagosome biogenesis: From membrane growth to closure

“…There are several examples of lipid binding and curvature sensing proteins involved in autophagy. Prime examples include the BAR domain-containing proteins SNX18 and BIF-1/Endophilin-1, as well as the fission yeast proteins Atg20 and Atg24 (Knaevelsrud et al, 2013;Takahashi et al, 2011;Zhao et al, 2016); the ATG8 conjugation machinery proteins ATG3 and ATG16L1, containing membrane inserted helixes essential for ATG8 lipidation (Lystad et al, 2019;Nath et al, 2014); and LC3 itself, being covalently conjugated to PE (Knorr et al, 2014). Moreover, several components of the ULK and PIK3C3 complexes contain specific regions that likely facilitate membrane recruitment in a geometry-dependent manner, including an EAT domain in Atg1/ULK1 (Chan et al, 2009) and a BATS domain in ATG14 (Fan et al, 2011).…”

Sensing Membrane Curvature in Macroautophagy

“…In particular, the Snx-BAR protein Snx18 drives LC3-positive tubule formation at recycling endosomes and also coordinates the Atg5-Atg12/Atg16 complex at these sites, suggesting local membrane tubulation might be involved in autophagosome biogenesis at this site 81, 83 . Snx-BAR proteins have also been implicated to act directly upon the phagophore itself 84 . Enrichment of LC3-PE or Atg8-PE at the phagophore rim has not been observed in fluorescence or immuno-EM studies, but modest enrichment would be challenging to detect as we expect a high concentration of the protein to decorate the inner membrane where it engages cargo and on the outer membrane where cytoplasmic proteins are engaged.…”

Independent losses and duplications of autophagy‐related genes in fungal tree of life

“…In S. cerevisiae, whereas Atg20 is required for both pexophagy and pexophagy-independent endosomal retrieval trafficking, Snx41 is only involved in the latter (Hettema et al, 2003;Deng et al, 2012). The two ATG20 paralogs also differ in their functions in S. pombe (Zhao et al, 2016). MoATG20, the only ATG20 ortholog in M. oryzae, was shown to play roles in both protein sorting (Snx41 function) and pexophagy (Atg20 function) (Deng et al, 2013).…”