Function of theSNAREYkt6 on autophagosomes requires the Dsl1 complex and the Atg1 kinase complex

Abstract

The mechanism and regulation of fusion between autophagosomes and lysosomes/vacuoles are still only partially understood in both yeast and mammals. In yeast, this fusion step requiresSNAREproteins, the homotypic vacuole fusion and protein sorting (HOPS) tethering complex, theRAB7GTPase Ypt7, and its guanine nucleotide exchange factor (GEF) Mon1-Ccz1. We and others recently identified Ykt6 as the autophagosomalSNAREprotein. However, it has not been resolved when and how lipid-anchored Ykt6 is recruited onto autophagosomes. Here, we show that Ykt6 is recruited at an early stage of the formation of these carriers through a mechanism that depends on endoplasmic reticulum (ER)-resident Dsl1 complex andCOPII-coated vesicles. Importantly, Ykt6 activity on autophagosomes is regulated by the Atg1 kinase complex, which inhibits Ykt6 through direct phosphorylation. Thus, our findings indicate that the Ykt6 pool on autophagosomal membranes is kept inactive by Atg1 phosphorylation, and once an autophagosome is ready to fuse with vacuole, Ykt6 dephosphorylation allows its engagement in the fusion event.