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Atg11 initiates selective autophagy in yeast by tethering Atg9 vesicles
Atg11 initiates selective autophagy in yeast by tethering Atg9 vesicles
Autophagy is a cellular recycling pathway that delivers material from the cytosol to the lysosomal lumen. In yeast, this process is initiated by the fusion of small vesicles to form a double-membrane sheet, the phagophore. The phagophore is continuously expanding and thereby engulfs its cytosolic cargo. Finally the membrane is sealed, giving rise to a double- membrane vesicle called autophagosome which fuses with the lysosome (or vacuole in yeast) to release its contents for degradation. Autophagy was initially discovered as response to starvation to ensure the cell’s survival by degradation of bulk cytosol. However, during nutrient-rich conditions autophagy is selectively capturing cargo, such as damaged or superfluous organelles or large protein aggregates. It is thus vital for cellular homeostasis and plays a role in the protection against cancer and neurodegenerative diseases. Autophagy is driven by a set of autophagy-related (Atg) gene products. The initiation of starvation-induced autophagy requires the transmembrane protein Atg9 and the Atg1 kinase complex. Atg9 is sorted from the Golgi to a dedicated set of small vesicles that are recruited to the site of autophagosome formation by the Atg1 kinase complex. The latter is activated in response to a starvation signal which induces the assembly of the complex. The active Atg1 kinase complex nucleates phagophore membranes by tethering Atg9 vesicles in order to prepare them for subsequent fusion. Atg17, the principle tethering subunit of the Atg1 kinase complex, is only active during starvation. How selective autophagy is initiated remains thus an open question. Several in vivo studies demonstrated that another autophagy-specific factor, Atg11, interacts with Atg9 and subunits of the Atg1 kinase complex during selective autophagy. However, the molecular function of this protein remained unclear. In the present thesis this question was addressed by reconstituting autophagy initiation from purified components in vitro. It could be shown that Atg11 binds to Atg9 reconstituted in liposomes and is able to tether such vesicles. However, this requires dimerization of Atg11 which is inhibited by its C-terminal domain. The activation of Atg11 occurs through binding of autophagic cargo. Moreover, Atg11 and Atg17 compete for binding of Atg9 proteoliposomes, but Atg17 is inactive under physiological conditions. This allows Atg11 to initiate selective autophagy. The cargo- dependent activation of Atg11 ensures that selective autophagy only occurs if cargo is present and spatiotemporally links the nucleation of the phagophore to such cargo. Starvation induces the degradation of Atg11 and activates Atg17, which nucleates phagophores independently of cargo.
Not available
Mayrhofer, Peter
2019
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Mayrhofer, Peter (2019): Atg11 initiates selective autophagy in yeast by tethering Atg9 vesicles. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

Autophagy is a cellular recycling pathway that delivers material from the cytosol to the lysosomal lumen. In yeast, this process is initiated by the fusion of small vesicles to form a double-membrane sheet, the phagophore. The phagophore is continuously expanding and thereby engulfs its cytosolic cargo. Finally the membrane is sealed, giving rise to a double- membrane vesicle called autophagosome which fuses with the lysosome (or vacuole in yeast) to release its contents for degradation. Autophagy was initially discovered as response to starvation to ensure the cell’s survival by degradation of bulk cytosol. However, during nutrient-rich conditions autophagy is selectively capturing cargo, such as damaged or superfluous organelles or large protein aggregates. It is thus vital for cellular homeostasis and plays a role in the protection against cancer and neurodegenerative diseases. Autophagy is driven by a set of autophagy-related (Atg) gene products. The initiation of starvation-induced autophagy requires the transmembrane protein Atg9 and the Atg1 kinase complex. Atg9 is sorted from the Golgi to a dedicated set of small vesicles that are recruited to the site of autophagosome formation by the Atg1 kinase complex. The latter is activated in response to a starvation signal which induces the assembly of the complex. The active Atg1 kinase complex nucleates phagophore membranes by tethering Atg9 vesicles in order to prepare them for subsequent fusion. Atg17, the principle tethering subunit of the Atg1 kinase complex, is only active during starvation. How selective autophagy is initiated remains thus an open question. Several in vivo studies demonstrated that another autophagy-specific factor, Atg11, interacts with Atg9 and subunits of the Atg1 kinase complex during selective autophagy. However, the molecular function of this protein remained unclear. In the present thesis this question was addressed by reconstituting autophagy initiation from purified components in vitro. It could be shown that Atg11 binds to Atg9 reconstituted in liposomes and is able to tether such vesicles. However, this requires dimerization of Atg11 which is inhibited by its C-terminal domain. The activation of Atg11 occurs through binding of autophagic cargo. Moreover, Atg11 and Atg17 compete for binding of Atg9 proteoliposomes, but Atg17 is inactive under physiological conditions. This allows Atg11 to initiate selective autophagy. The cargo- dependent activation of Atg11 ensures that selective autophagy only occurs if cargo is present and spatiotemporally links the nucleation of the phagophore to such cargo. Starvation induces the degradation of Atg11 and activates Atg17, which nucleates phagophores independently of cargo.