The mechanism of Atg15-mediated membrane disruption in autophagy
Autophagy is a lysosomal/vacuolar delivery system that isolates and degrades cytoplasmic material. Following delivery by autophagosomes, cytoplasmic components are released into the vacuole within an autophagic body (AB), which is a single-membrane structure derived from the inner membrane of the autophagosome. This membrane must be disrupted for degradation of the cytoplasmic cargo to occur. The vacuolar proteases Pep4 and Prb1, as well as the lipase Atg15, are known to be necessary for this process, but the mechanistic underpinnings remain unclear. In this study, we establish a system to detect lipase activity in the vacuole and use it to show that Atg15 is the sole vacuolar phospholipase and that Pep4 and Prb1 are required for the activation of Atg15 lipase function, which occurs following delivery of Atg15 to the vacuole by the MVB pathway. In vitro experiments also reveal that Atg15 is a B-type phospholipase of broad substrate specificity that is likely implicated in the disruption of a range of membranes delivered to the vacuole. Further, we use isolated ABs to demonstrate that Atg15 alone is able to disrupt AB membranes.