UC San Diego

Macroautophagy is a process of bulk degradation initiated during cellular starvation or stress. When macroautophagy (hereafter referred to as autophagy) is induced there is a rapid upregulation in the formation of autophagosomes; double membrane vesicles, which engulf cytoplasmic materials and organelles. After formation, autophagosomes fuse with the vacuole releasing their contents for degradation, which is essential to maintain homeostasis during nutrient deprivation. Although the membrane source and trafficking events leading to autophagosome formation are poorly understood, recent findings have implicated endoplasmic reticulum (ER)-derived COPII vesicles in autophagosome biogenesis. During nutrient rich conditions, COPII vesicles work on the secretory pathway by transporting cargo from the ER to the Golgi. How the function of COPII vesicles is regulated to balance their roles in secretion and autophagy is unclear. Moreover, the molecular mechanism of how COPII vesicles recognize autophagy machinery to aid in autophagosome formation is unknown. The current understanding of the function of COPII vesicles and other parts of the secretory pathway in autophagy is discussed in Chapter 1.

The work in this dissertation examines how phosphorylation of Sec24, a subunit of the inner COPII coat, regulates autophagosome formation. In Chapter 2 we rule out a major role for Sec24 phosphorylation in regulating the secretory pathway. We next screened for Sec24 phosphorylation sites that are required for autophagy and not ER-Golgi transport. This analysis identified a patch of conserved phosphorylation sites that are required for autophagosome formation during starvation. Phosphorylation of these residues regulates autophagosome frequency and enhances the interaction of the COPII coat with Atg9, a key regulator of autophagosome initiation. Chapter 3 discusses the role of the serine/threonine kinase Hrr25 in regulating the Sec24-Atg9 interaction through Sec24 phosphorylation. Chapter 4 discusses the interacting domains of Uso1, a long coil-coiled tether that links COPII vesicles to the Golgi on the secretory pathway. Chapter 5 places these findings into the broader context of COPII vesicle trafficking and autophagosome formation. Additionally, this final chapter gives recommendations for future experiments to further our knowledge of the membrane rearrangements required for autophagosome initiation.