Roles of vacuolar proteases in the autophagic degradation of nuclei and other substrates in Aspergillus oryzae

Background

We previously reported that in the filamentous fungus Aspergillus oryzae entire nuclei were degraded by autophagy when cells are exposed to nutrient starvation. In this study we investigated the role of vacuolar proteases in autophagic degradation of nuclei (nucleophagy) and bulk autophagy in A. oryzae. Methods and results: We constructed A. oryzae mutants disrupted for pepE and Aoprb1 encoding the orthologs of major vacuolar proteases Pep4 and Prb1, respectively, in Saccharomyces cerevisiae. Surprisingly, disruption of pepE did not result in any autophagy-related phenotypic alterations such as reduced conidiation and decrease in starvation-induced substrate degradation. In contrast, Aoprb1 disruptant exhibited apparently contradictory phenotypes: while conidiation was severely impaired and accumulation of autophagic bodies in the vacuoles was observed, degradation of AoAtg8 and histone H2B, markers for autophagic flux and nucleophagy, respectively, only slightly or not significantly reduced. Double disruption of pepE and Aoprb1 markedly impaired AoAtg8 degradation, whereas nucleophagy was only partially reduced.