Investigating the function of Gdt1p in yeast Golgi glycosylation

The Golgi ion homeostasis is tightly regulated to ensure essential cellular processes such as glycosylation, yet our understanding of this regulation remains incomplete. Gdt1p is a member of the conserved Uncharacterized Protein Family (UPF0016). Our previous work suggested that Gdt1p may function in the Golgi by regulating Golgi Ca²⁺/Mn²⁺ homeostasis. NMR structural analysis of the polymannan chains isolated from yeasts showed that the gdt1Δ mutant cultured in presence of high Ca²⁺ concentration, as well as the pmr1Δ and gdt1Δ/pmr1Δ strains presented strong late Golgi glycosylation defects with a lack of α-1,2 mannoses substitution and α-1,3 mannoses termination. The addition of Mn²⁺ confirmed the rescue of these defects. Interestingly, our structural data confirmed that the glycosylation defect in pmr1Δ could also completely be suppressed by the addition of Ca²⁺. The use of Pmr1p mutants either defective for Ca²⁺ or Mn²⁺ transport or both revealed that the suppression of the observed glycosylation defect in pmr1Δ strains by the intraluminal Golgi Ca²⁺ requires the activity of Gdt1p. These data support the hypothesis that Gdt1p, in order to sustain the Golgi glycosylation process, imports Mn²⁺ inside the Golgi lumen when Pmr1p exclusively transports Ca²⁺. Our results also reinforce the functional link between Gdt1p and Pmr1p as we highlighted that Gdt1p was a Mn²⁺ sensitive protein whose abundance was directly dependent on the nature of the ion transported by Pmr1p. Finally, this study demonstrated that the aspartic residues of the two conserved motifs E-x-G-D-[KR], likely constituting the cation binding sites of Gdt1p, play a crucial role in Golgi glycosylation and hence in Mn²⁺/Ca²⁺transport.