An Atg13 Protein-mediated Self-association of the Atg1 Protein Kinase Is Important for the Induction of Autophagy

Autophagy pathways in eukaryotic cells mediate the turnover of a diverse set of cytoplasmic components, including damaged organelles and abnormal protein aggregates. Autophagy-mediated degradation is highly regulated, and defects in these pathways have been linked to a number of human disorders. The Atg1 protein kinase appears to be a key site of this control and is targeted by multiple signaling pathways to ensure the appropriate autophagic response to changing environmental conditions. Despite the importance of this kinase, relatively little is known about the molecular details of Atg1 activation. In this study we show that Atg13, an evolutionarily conserved regulator of Atg1, promotes the formation of a specific Atg1 self-interaction in the budding yeast, Saccharomyces cerevisiae. The appearance of this Atg1-Atg1 complex is correlated with the induction of autophagy, and conditions that disrupt this complex result in diminished levels of both autophagy and Atg1 kinase activity. Moreover, the addition of a heterologous dimerization domain to Atg1 resulted in elevated kinase activity both in vivo and in vitro. The formation of this complex appears to be an important prerequisite for the subsequent autophosphorylation of Thr-226 in the Atg1 activation loop. Previous work indicates that this modification is necessary and perhaps sufficient for Atg1 kinase activity. Interestingly, this Atg1 self-association does not require Atg17, suggesting that this second conserved regulator might activate Atg1 in a manner mechanistically distinct from that of Atg13. In all, this work suggests a model whereby this self-association stimulates the autophosphorylation of Atg1 within its activation loop.