Killer Toxin K28 resistance in yeast relies on COG complex mediated trafficking of the defence factor Ktd1

A/B toxins are a diverse family of protein toxins that enter host cells by endocytosis and trigger cell death. In yeast, the killer toxin K28 is an A/B family member that is internalised to endosomes of susceptible host yeast, where the toxin follows the retrograde trafficking pathway and ultimately induces cell cycle arrest. Yeast cells that express the endolysosomal Ktd1 defence factor are protected from K28 toxicity, but modes of Ktd1 regulation are not understood. The COG complex is a conserved tethering complex that is involved in membrane trafficking at the Golgi. Previous research implicated Cog7, a subunit of the COG complex, in K28 defence, but the mechanism is unknown. We employed a high throughput K28 sensitivity assay and bespoke analysis package to show that all lobe B subunits (Cog5 - 8) are required for K28 resistance. Deletion of Golgi localised glycosyltransferase enzymes provides resistance to K28, as cells lack the glycosylated surface proteins required for K28 binding and entry. Although the COG complex modulates glycosylation of surface molecules, our experiments reveal that the hypersensitivity of cog mutants is not due to changes in K28 binding but to defects in intracellular trafficking. Ktd1 is mis-localised in cog mutants, with a trafficking defect that parallels similar disruptions observed with Snc1, a surface cargo protein that is dependent on the COG complex for recycling via the Golgi. These findings suggest that the COG complex ensures proper trafficking of Ktd1 to endolysosomal compartments, where it can effectively neutralise K28. We propose a model whereby Ktd1 needs to be precisely localised to survey endolysosomes and effectively mediate K28 defence. This work underpins the importance of Ktd1 in defence against the A/B toxin K28, and implies various membrane trafficking regulators might regulate toxin effects in other eukaryotic systems.