Abstract
During autophagy, a newly formed double membrane surrounds its cargo to generate the so-called autophagosome, which then fuses with a lysosome after closure. Previous work implicated that endosomal Rab7/Ypt7 associates to autophagosomes prior to their fusion with lysosomes. Here, we unravel how the Mon1-Ccz1 guanosine exchange factor (GEF) acting upstream of Ypt7 is specifically recruited to the pre-autophagosomal structure under starvation conditions. We find that Mon1-Ccz1 directly binds to Atg8, the yeast homolog of the members of the mammalian LC3 protein family. This requires at least one LIR motif in the Ccz1 C-terminus, which is essential for autophagy but not for endosomal transport. In agreement, only wild-type, but not LIR-mutated Mon1-Ccz1 promotes Atg8-dependent activation of Ypt7. Our data reveal how GEF targeting can specify the fate of a newly formed organelle and provide new insights into the regulation of autophagosome-lysosome fusion.
Highlights
Macroautophagy, called here autophagy, is an important quality control pathway, during which cellular material such as organelles and cytosolic components are engulfed by a double-membrane vesicles, the autophagosome (Shibutani and Yoshimori, 2014; Mizushima et al, 2011)
To determine how yeast autophagosomes are decorated with Ypt7, we analyzed the subcellular distribution of both Mon1 and Ccz1 as the guanine nucleotide exchange factor (GEF) complex formed by these two proteins (Nordmann et al, 2010)
Deletion of the vacuolar Qa-SNARE Vam3, or temperature sensitive mutants of either Vam3 or the HOPS subunit Vps11 block fusion processes with the vacuole (Darsow et al, 1997; Peterson and Emr, 2001)
Summary
Macroautophagy, called here autophagy, is an important quality control pathway, during which cellular material such as organelles and cytosolic components are engulfed by a double-membrane vesicles, the autophagosome (Shibutani and Yoshimori, 2014; Mizushima et al, 2011). In both yeast and mammals, autophagosome formation is a complex process that begins with the assembly of the phagophore or isolation membrane. Like for other fusion events, autophagosome fusion with vacuoles or endosomes requires SNAREs, Rab GTPases (Rabs) and the HOPS tethering complex (Reggiori and Ungermann, 2017; Barr, 2013; Kummel and Ungermann, 2014). Rab as well as Ypt are required for fusion of autophagosomes with endosomes
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