TMEM165 deficiencies lead to one of the Congenital Disorders of Glycosylation (CDG), a group of inherited diseases where the glycosylation process is altered. We recently demonstrated that the Golgi glycosylation defect due to TMEM165 deficiency resulted from Golgi manganese homeostasis defect and that Mn2+ supplementation was sufficient to rescue a normal glycosylation. In this paper we highlight TMEM165 as a novel Golgi protein sensitive to manganese. When cells were exposed to high Mn2+ concentrations, TMEM165 was degraded into lysosomes. The Mn2+ induced lysosomal targeting of TMEM165 occurred via a Rab7 and Rab5 independent pathways, suggesting a direct trafficking from the Golgi to lysosomes in response to Mn2+. Remarkably, the variant p.E108G recently identified in a novel TMEM165-CDG patient, was found insensitive to Mn2+ supplementation. Moreover, this mutation abolished the function of TMEM165, suggesting that a transport function may be necessary for its regulation. Altogether our results identified the Golgi protein TMEM165 as a novel cytosolic Mn2+ sensor in mammalian cells and pointed to the crucial importance of the cytosolical ELGDK motif in both Mn2+ sensitivity and function.

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 Ca/Mn homeostasis. NMR structural analysis of the polymannan chains isolated from yeasts showed that the gdt1Δ mutant cultured in presence of high Ca 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 Mn 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 Ca. The use of Pmr1p mutants either defective for Ca or Mn transport or both revealed that the suppression of the observed glycosylation defect in pmr1Δ strains by the intraluminal Golgi Ca requires the activity of Gdt1p. These data support the hypothesis that Gdt1p, in order to sustain the Golgi glycosylation process, imports Mn inside the Golgi lumen when Pmr1p exclusively transports Ca. Our results also reinforce the functional link between Gdt1p and Pmr1p as we highlighted that Gdt1p was a Mn 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 Mn/Catransport.

N-linked glycosylation interference and glycolysis disruption both contribute to the anticancer properties of 2-DG in breast cancer cells.

Since 2012, the interest for TMEM165 increased due to its implication in a rare genetic human disease named TMEM165-CDG (Congenital Disorder(s) of Glycosylation). TMEM165 is a Golgi localized protein, highly conserved through evolution and belonging to the uncharacterized protein family 0016 (UPF0016). Although the precise function of TMEM165 in glycosylation is still controversial, our results highly suggest that TMEM165 would act as a Golgi Ca2+/Mn2+ transporter regulating both Ca2+ and Mn2+ Golgi homeostasis, the latter is required as a major cofactor of many Golgi glycosylation enzymes. Strikingly, we recently demonstrated that besides its role in regulating Golgi Mn2+ homeostasis and consequently Golgi glycosylation, TMEM165 is sensitive to high manganese exposure. Members of the UPF0016 family contain two particularly highly conserved consensus motifs E-φ-G-D-[KR]-[TS] predicted to be involved in the ion transport function of UPF0016 members. We investigate the contribution of these two specific motifs in the function of TMEM165 in Golgi glycosylation and in its Mn2+ sensitivity. Our results show the crucial importance of these two conserved motifs and underline the contribution of some specific amino acids in both Golgi glycosylation and Mn2+ sensitivity.

Congenital disorders of glycosylation are severe inherited diseases in which aberrant protein glycosylation is a hallmark. Transmembrane protein 165 (TMEM165) is a novel Golgi transmembrane protein involved in type II congenital disorders of glycosylation. Although its biologic function is still a controversial issue, we have demonstrated that the Golgi glycosylation defect due to TMEM165 deficiency resulted from a Golgi Mn2+ homeostasis defect. The goal of this study was to delineate the cellular pathway by which extracellular Mn2+ rescues N‐glycosylation in TMEM165 knockout (KO) cells. We first demonstrated that after extracellular exposure, Mn2+ uptake by HEK293 cells at the plasma membrane did not rely on endocytosis but was likely done by plasma membrane transporters. Second, we showed that the secretory pathway Ca2+‐ATPase 1, also known to mediate the influx of cytosolic Mn2+ into the lumen of the Golgi apparatus, is not crucial for the Mn2+‐induced rescue glycosylation of lysosomal‐associated membrane protein 2 (LAMP2). In contrast, our results demonstrate the involvement of cyclopiazonic acid—and thapsigargin (Tg)‐sensitive pumps in the rescue of TMEM165‐associated glycosylation defects by Mn2+. Interestingly, overexpression of sarco/endoplasmic reticulum Ca2+‐ATPase (SERCA) 2b isoform in TMEM165 KO cells partially rescues the observed LAMP2 glycosylation defect. Overall, this study indicates that the rescue of Golgi N‐glycosylation defects in TMEM165 KO cells by extracellular Mn2+ involves the activity of Tg and cyclopiazonic acid–sensitive pumps, probably the SERCA pumps.—Houdou, M., Lebredonchel, E., Garat, A., Duvet, S., Legrand, D., Decool, V., Klein, A., Ouzzine, M., Gasnier, B., Potelle, S., Foulquier, F. Involvement of thapsigargin—and cyclopiazonic acid–sensitive pumps in the rescue of TMEM165‐associated glycosylation defects by Mn2+. FASEB J. 33, 2669–2679 (2019). http://www.fasebj.org

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TMEM165 was highlighted in 2012 as the first member of the Uncharacterized Protein Family 0016 (UPF0016) related to human glycosylation diseases. Defects in TMEM165 are associated with strong Golgi glycosylation abnormalities. Our previous work has shown that TMEM165 rapidly degrades with supraphysiological manganese supplementation. In this paper, we establish a functional link between TMEM165 and SPCA1, the Golgi Ca2+/Mn2+ P-type ATPase pump. A nearly complete loss of TMEM165 was observed in SPCA1-deficient Hap1 cells. We demonstrate that TMEM165 was constitutively degraded in lysosomes in the absence of SPCA1. Complementation studies showed that TMEM165 abundance was directly dependent on SPCA1's function and more specifically its capacity to pump Mn2+ from the cytosol into the Golgi lumen. Among SPCA1 mutants that differentially impair Mn2+ and Ca2+ transport, only the Q747A mutant that favors Mn2+ pumping rescues the abundance and Golgi subcellular localization of TMEM165. Interestingly, the overexpression of SERCA2b also rescues the expression of TMEM165. Finally, this paper highlights that TMEM165 expression is linked to the function of SPCA1.

N‐glycosylation is an important post‐translational modification of proteins that has been highly conserved during evolution and is found in Eukaryota, Bacteria and Archaea. In eukaryotes, N‐glycan processing is sequential, involving multiple specific steps within the secretory pathway as proteins travel through the endoplasmic reticulum and the Golgi apparatus. In this review, we first summarize the different steps of the N‐glycan processing and further describe recent findings regarding the diversity of N‐glycan structures in eukaryotic clades. This comparison allows us to explore the different regulation mechanisms of N‐glycan processing among eukaryotic clades. Recent findings regarding the regulation of protein N‐glycosylation are highlighted, especially the regulation of the biosynthesis of complex‐type N‐glycans through manganese and calcium homeostasis and the specific role of transmembrane protein 165 (TMEM165) for which homologous sequences have been identified in several eukaryotic clades. Further research will be required to characterize the function of TMEM165 homologous sequences in different eukaryotic clades.