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Source: https://theses.gla.ac.uk/5046/ Parent: https://theses.gla.ac.uk/view/subjects/QH345.html

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A role for the endosomal SNARE complex and tethers in autophagy

Cowan, Marianne (2014) A role for the endosomal SNARE complex and tethers in autophagy. PhD thesis, University of Glasgow.

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Printed Thesis Information: https://eleanor.lib.gla.ac.uk/record=b3031957

Abstract

Autophagy is a major route for lysosomal and vacuolar degradation in mammals and yeast respectively. It is involved in diverse physiological processes and implicated in numerous pathologies. The process of autophagy is initiated at the pre-autophagosomal structure and is characterised by the formation of a double membrane vesicle termed the autophagosome which sequesters cytosolic components and targets them for lysosomal/vacuolar degradation. The molecular mechanisms that regulate autophagosome formation are not fully understood. The conserved oligomeric Golgi (COG) complex is a hetero-octameric tethering factor implicated in autophagosome formation which interacts directly with the target membrane SNARE proteins Syntaxin 6 and Syntaxin 16 via the Cog6 and Cog4 subunits respectively. The work presented in this thesis demonstrates direct interaction of the yeast orthologue of Syntaxin 16, Tlg2, with Cog2 and Cog4. In addition, I investigated binding of the COG complex subunits to Tlg1, Vti1 and Snc2, the partner SNARE proteins of Tlg2. Direct interaction of Tlg1, the yeast orthologue of Syntaxin 6, with Cog1, Cog2 and Cog4 were observed. Given that Tlg2 has previously been shown to regulate autophagy in yeast, these data support a conserved role for the COG complex in mediating autophagosome formation through regulation of SNARE complex formation.\ In addition to investigating binding of COG complex subunits to the endosomal SNARE complex, I have also investigated a role for autophagy in regulating Tlg2 levels. The SM protein Vps45 has previously been shown to stabilise Tlg2 cellular levels. Our laboratory has demonstrated a role for both the proteasome and vacuole in the degradation of Tlg2. Here I demonstrated a role for autophagy in the regulation of Tlg2 levels and show that Swf1-mediated palmitoylation may serve to protect Tlg2 from being selectively targeted for autophagy. I also investigated the effects of the yeast T238N mutation on Vps45 function. The analogous mutation in human Vps45 has recently been associated with congenital neutropenia. Vps45 function is best characterised in yeast where it associates with membranes via Tlg2 and is required for membrane traffic from the trans-Golgi network into the endosomal system. Cellular levels of Vps45 T238N were destabilised and a concomitant reduction in Tlg2 levels was also observed. Vacuolar protein sorting remained unaffected in yeast cells harboring Vps45 T238N but was subjected to increased apoptosis under hydrogen peroxide-mediated stress. This identifies a novel role for Vps45 in maintaining cell viability. Finally, I also investigated a role for endosomal trafficking and autophagy in C.elegans post-embryonic development and identified a role for these pathways in the clearance of the pre-moult increase in intracellular membranes and cuticular formation.

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