Autophagic and non-autophagic functions of the S. cerevisiae PROPPINs Atg18, Atg21 and Hsv2

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Atg18, Atg21 and Hsv2 are yeast members of a highly conserved family of PROPPINs (-propeller that bind polyphosphoinositides). They bind via two binding sites at their circumference to PI3P and PI(3,5)P2. Atg18 and Atg21 were initially identified for their requirement for macroautophagy (hereafter autophagy). The function of the homologous Hsv2 remained elusive. We only found a partial effect on micronucleophagy, where the contact site between the vacuole and the nuclear envelope is degraded during nitrogen starvation together with non-essential parts of the nucleus. PROPPINs have beside their PI3P-dependent roles in autophagy additional non-autophagic functions. Atg18 negatively regulates the vacuolar PI3P 5-kinase Fab1. It further functions in retrograde transport of the artificial cargo RS-ALP from the vacuole to the Golgi. Atg18 and to a lesser extend Atg21 are further involved in vacuolar fragmentation upon hyperosmotic shock. To better understand the molecular functions of the PROPPINs, we used the proximity labeling BioID approach for all three PROPPINs, which allows in living cells the identification of proteins within the microenvironment of the bait proteins. In an already published study we found that Atg18 is part of a novel retromer complex, which mediates vacuole fragmentation upon hyperosmotic stress. We further showed that the cycling of the integral membrane protein Atg9, which is essential for autophagosome biogenesis depends on Atg18-retromer. Using two types of Atg18 mutants, which are expected to be selectively impaired in its role in protein recycling but not autophagy, we now want to understand the precise molecular function of Atg18-retromer. Further major aims are to clarify the function of Hsv2 and to dissect the organization of Atg21-complexes at the phagophore. To this end, we will carry out in depth analyses of a set of novel PROPPIN interactors found in our unpublished proximity labeling screens.