The role of selective autophagy in growth and development of filamentous fungi

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In filamentous ascomycetes, autophagy is involved in various developmental processes. Two types of autophagy have been described: non-selective and selective autophagy. Non-selective autophagy is the random engulfment of cytoplasm and organelles into double-membrane vesicles, the autophagosomes, which deliver the cargo to the vacuole for degradation. In selective autophagy, specific cargos such as organelles, protein aggregates or enzymes are recognized by cargo receptors and enwrapped into autophagosomes. For sexual propagation, filamentous fungi produce three-dimensional fruiting bodies, where the sexual ascospores are generated. The homothallic (self-fertile) ascomycete Sordaria macrospora is an excellent fungal model organism to study multicellular fruiting-body development. In recent years, molecular genetics procedures have been established to isolate and characterize developmental genes in S. macrospora; however, the exact role of autophagy in multicellular fruiting-body development is largely unknown.Using a reverse genetics approach, we have previously shown that conserved genes required for core functions of the selective and non-selective autophagic machinery are essential for fruiting-body development in S. macrospora. We used GFP-Trap analysis with the ubiquitin-like membrane associated EGFP-tagged SmATG8 to identify interacting proteins. Our study identified 17 proteins as putative SmATG8 interaction partners, among these a putative homolog of the mammalian selective cargo receptor Neighbor of BRCA1 (NBR1). In mammals and plants, NBR1 was shown to act as cargo receptor or adaptor for the autophagic degradation of ubiquitinated substrates and the degradation of peroxisomes. A homolog of NBR1 is not encoded in Saccharomyces cerevisiae; therefore S. macrospora is a useful model to study the functions of this autophagy receptor. Two-hybrid experiments, co-immunoprecipitation and in vivo co-localization revealed that SmNBR1 and SmATG8 can directly interact with each other. In addition, we could show that SmNBR1 is transported to the vacuole in a SmATG8-dependent manner. A DeltaSmnbr1 mutant revealed a decreased growth rate under starvation conditions and a delayed sexual development as well as impairment in ascospore formation and pexophagy. By studying the interaction of both proteins and identifying new interaction partners of SmATG8 and SmNBR1, we aim to understand how selective autophagy contributes to vegetative growth and development in fungi.