Pexophagy and fruiting-body formation in Sordaria macrospora

The homothallic filamentous ascomycete Sordaria macrospora is an ideal model organism to study multicellular fruiting-body development. The fungus lacks asexual reproduction and is strictly dependent on the sexual cycle for the production of ascospores.

Supply and homeostasis of nutrients are important issues for sexual development. We therefore analyzed the role of non-selective autophagy during fruiting-body formation. Autophagy is a degradation process in which eukaryotic cells digest their own cell constituents. We have characterized conserved components of the autophagic machinery and could show that autophagy is an essential and constitutively active process to sustain high energy levels for multicellular development. In contrast to non-selective bulk autophagy, selective autophagy is characterized by cargo receptors, which bind specific cargos such as superfluous or damaged organelles, and target them for autophagic degradation. Using the core autophagy protein ATG8 as bait, GFP-Trap analysis followed by liquid chromatography mass spectrometry (LC/MS) identified a putative homolog of the human autophagy cargo receptor neighbour of BRCA1 (NBR1) in S. macrospora. Fluorescence microscopy revealed that SmNBR1 co-localizes with SmATG8 at autophagosome-like structures and in the lumen of vacuoles. Delivery of SmNBR1 to the vacuoles requires SmATG8. Both proteins interact in an LC3 interacting region (LIR)-dependent manner. Deletion of Smnbr1 leads to impaired vegetative growth under starvation conditions, and reduced sexual spore production under non-starvation conditions. The human nbr1 homolog partially rescues the phenotypic defects of the fungal Smnbr1 deletion mutant. The Smnbr1 mutant can neither use fatty acids as a sole carbon source, nor form fruiting bodies under oxidative stress conditions. Fluorescence microscopy revealed that degradation of a peroxisomal reporter protein is impaired in the Smnbr1 deletion mutant. Thus, SmNBR1 is a cargo receptor for peroxisomes in filamentous ascomycetes.