Atg15 is a vacuolar phospholipase that disintegrates organelle membranes

Atg15 (autophagy-related 15) is a vacuolar phospholipase essential for the degradation of cytoplasm-to-vacuole targeting (Cvt) bodies and autophagic bodies, hereinafter referred to as intravacuolar/intralysosomal autophagic compartments (IACs), but it remains unknown if Atg15 directly disrupts IAC membranes. Here, we show that the recombinant Chaetomium thermophilum Atg15 lipase domain (CtAtg15(73-475)) possesses phospholipase activity. The activity of CtAtg15(73-475) was markedly elevated by limited digestion. We inserted the human rhinovirus 3C protease recognition sequence and found that cleavage between S159 and V160 was important to activate CtAtg15(73-475). Our molecular dynamics simulation suggested that the cleavage facilitated conformational change around the active center of CtAtg15, resulting in an exposed state. We confirmed that CtAtg15 could disintegrate S. cerevisiae IAC in vivo. Further, both mitochondria and IAC of S. cerevisiae were disintegrated by CtAtg15. This study suggests Atg15 plays a role in disrupting any organelle membranes delivered to vacuoles by autophagy.

TRAPPIII requires Drs2 binding to transport Atg9 vesicles at cold temperatures

ABBREVIATIONS

Autophagy-related 9 (Atg9); cytoplasm-to-vacuole targeting (Cvt); Golgi-associated retrograde protein (GARP); multisubunit tethering complexes (MTCs); phagophore assembly site (PAS); phosphatidylserine (PS); Protein interactions from Imaging Complexes after Translocation (PICT); transport protein particle III (TRAPPIII); type IV P-type ATPases (P4-ATPases).

Atg23 is a vesicle-tethering protein

Small 30-nm vesicles containing the integral membrane protein Atg9 provide the initial membrane source for autophagy in yeast. Atg23, is an Atg9 binding protein that is required for Atg9 vesicle trafficking but whose exact function is unknown. In our recent paper, we explored the function of Atg23 using an approach combining cellular biology and biochemistry on purified protein. We determined that Atg23 is an elongated dimer spanning 320 Å in length. We also demonstrated that Atg23 is a membrane-binding and -tethering protein. Furthermore, we identified a series of amino acids residing in a putative coiled- coil region that when mutated prevent Atg23 dimer formation resulting in a stable Atg23 monomer. Last, we demonstrated that when monomeric Atg23 is expressed in yeast lacking Atg23, this leads to a loss of Atg23 puncta, a reduction in Atg9 puncta, a reduction in non-selective autophagy and a complete block in the cytoplasm-to-vacuole targeting (Cvt) pathway.

Dissecting the role of the Atg12-Atg5-Atg16 complex during autophagosome formation

The activity of the conserved Atg12-Atg5-Atg16 complex is essential for autophagosome formation. However, little is known about its mechanism of action during this process. In our study we employed in vitro systems consisting of purified proteins and giant unilamellar vesicles (GUVs) or small liposomes to investigate membrane binding by the Atg12-Atg5-Atg16 complex and its interplay with the Atg8 conjugation system. We showed that Atg5 directly binds membranes and that this membrane binding is negatively regulated by Atg12 conjugation but activated by Atg16. Membrane binding by the Atg12-Atg5-Atg16 complex is required for efficient promotion of Atg8 lipidation. Additionally, we found that the Atg12-Atg5-Atg16 complex tethered vesicles in an Atg8-independent manner. In yeast, membrane binding by Atg5 is not required for its recruitment to the phagophore assembly site (PAS) but is essential for efficient promotion of autophagy and the cytoplasm-to-vacuole targeting (Cvt) pathway at a stage preceding Atg8 lipidation and autophagosome closure. Our findings provide new insights into the role of the Atg12-Atg5-Atg16 complex during autophagosome formation.