Proteomic and Phosphoryproteomic Investigations Reveal that Autophagy-Related Protein 1, a Protein Kinase for Autophagy Initiation, Synchronously Deploys Phosphoregulation on the Ubiquitin-Like Conjugation System in the Mycopathogen Beauveria bassiana

Autophagy is a conserved intracellular degradation mechanism in eukaryotes and is initiated by the protein kinase autophagy-related protein 1 (Atg1). However, except for the autophosphorylation activity of Atg1, the target proteins phosphorylated by Atg1 are largely unknown in filamentous fungi. In Beauveria bassiana (a filamentous insect-pathogenic fungus), Atg1 is indispensable for autophagy and is associated with fungal development. Comparative omics-based analyses revealed that B. bassiana Atg1 (BbAtg1) has key influence on the proteome and phosphoproteome during conidiogenesis. In terms of its physiological functions, the BbAtg1-mediated phosphoproteome is primarily associated with metabolism, signal transduction, cell cycle, and autophagy. At the proteomic level, BbAtg1 mainly regulates genes involved in protein synthesis, protein fate, and protein with binding function. Furthermore, integrative analyses of phosphoproteomic and proteomic data led to the identification of several potential targets regulated by BbAtg1 phosphorylation activity. Notably, we demonstrated that BbAtg1 phosphorylated BbAtg3, an essential component of the ubiquitin-like conjugation system in autophagic progress. Our findings indicate that in addition to being a critical component of the autophagy initiation, Atg1 orchestrates autophagosome elongation via its phosphorylation activity. The data from our study will facilitate future studies on the noncanonical targets of Atg1 and help decipher the Atg1-mediated phosphorylation networks. IMPORTANCE Autophagy-related protein 1 (Atg1) is a serine/threonine protein kinase for autophagy initiation. In contrast to the unicellular yeast, the target proteins phosphorylated by Atg1 are largely unknown in filamentous fungi. In this study, the entomopathogenic fungus Beauveria bassiana was used as a representative of filamentous fungi due to its importance in the applied and fundamental research. We revealed that Atg1 mediates the comprehensive proteome and phosphoproteome, which differ from those revealed in yeast. Further investigation revealed that Atg1 directly phosphorylates the E2-like enzyme Atg3 of the ubiquitin-like conjugation system (ULCS), and the phosphorylation of Atg3 is indispensable for ULCS functionality. Interestingly, the phosphorylation site of Atg3 is conserved among a set of insect- and plant-pathogenic fungi but not in human-pathogenic fungi. This study reveals new regulatory mechanisms of autophagy and provides new insights into the evolutionary diversity of the Atg1 kinase signaling pathways among different pathogenic fungi.