Study of the degradation mechanism of the yeast Gdt1p transporter

The UPF0016 protein family has been recently identified as a family of calcium and manganese transporters. TMEM165, the human UPF0016 member, is a Golgi membrane protein that is involved in protein glycosylation and in the maintenance of Golgi morphology. Mutations in TMEM165 can cause a congenital disorder of glycosylation, which is an inherited disease with various symptoms. Gdt1p is the yeast UPF0016 member, ortholog to TMEM165. It is also located in the Golgi membrane and plays a role in glycosylation. Both proteins are involved in manganese and calcium homeostasis, and they are degraded when cells are exposed to high manganese concentrations. In this project we tried to shed light on the mechanism underlying the manganese-induced degradation of Gdt1p. We revealed the inverse correlation between Gdt1p abundance and whole cell manganese content, linked with the ability of Pmr1p to import manganese into the Golgi. When Pmr1p does not transport manganese, it can not be extruded through the secretory pathway and accumulates in the cytoplasm. Manganese detoxification occurs by storing the Mn2+ ions in the vacuole. We also compared manganese and calcium distribution in the different organelles depending on the absence of Pmr1p and/or Gdt1p and on cellular manganese exposure. We showed that Gdt1p does not have a big influence on the ion distribution, even in the absence of Pmr1p. Finally we investigated if ubiquitylation is the signal targeting Gdt1p to the vacuole. After manganese exposure, we observed by western blot the apparition of an upper band that might correspond to ubiquitylated Gdt1p. However, when we purified the protein and analysed putative ubiquitylation with antibodies directed against ubiquitin, no signal was detected. It would be of interest to keep investigating the signal targeting Gdt1p to degradation by other methods.