New Toolset of Reporters Reveals That Glycogen Granules Are Neutral Substrates of Bulk Autophagy in Komagataella phaffii

Glycogen, a branched polysaccharide organized into glycogen granules (GGs), is delivered from the cytoplasm to the lysosomes of hepatocytes by STBD1-driven selective autophagy (glycophagy). Recently, we developed yeast as a simple model of GG autophagy and found that it proceeds non-selectively under nitrogen starvation conditions. However, another group, using as a model, found that glycogen is a non-preferred cargo of nitrogen starvation-induced bulk autophagy. To clarify cargo characteristics of GGs, we used the same glycogen synthase-based reporter (Gsy1-GFP) of GG autophagy in as was used in . The Gsy1-GFP marked the GGs and reported on their autophagic degradation during nitrogen starvation, as expected. However, unlike in , glycogen synthase-marked GGs were delivered to the vacuole and degraded there with the same efficiency as a cytosolic glycogen synthase in glycogen-deficient cells, suggesting that glycogen is a neutral cargo of bulk autophagy in . We verified our findings with a new set of reporters based on the glycogen-binding CBM20 domain of human STBD1. The GFP-CBM20 and mCherry-CBM20 fusion proteins tagged GGs, reported about the autophagy of GGs, and confirmed that GGs in are neither preferred nor non-preferred substrates of bulk autophagy. They are its neutral substrates.