Insights into molecular and cellular functions of the Golgi calcium/manganese-proton antiporter TMEM165

Abstract

The Golgi compartment performs a number of crucial roles in the cell. However, the exact molecular mechanisms underlying these actions are not fully defined. Pathogenic mutations in genes encoding Golgi proteins may serve as an important source for expanding our knowledge. For instance, mutations in the gene encoding Transmembrane protein 165 (TMEM165) were discovered as a cause of a new type of congenital disorder of glycosylation (CDG). Comprehensive studies of TMEM165 in different model systems, including mammals, yeast, and fish uncovered the new realm of Mn²⁺ homeostasis regulation. TMEM165 was shown to act as a Ca²⁺/Mn²⁺:H⁺ antiporter in the medial- and trans-Golgi network, pumping the metal ions into the Golgi lumen and protons outside. Disruption of TMEM165 antiporter activity results in defects in N- and O-glycosylation of proteins and glycosylation of lipids. Impaired glycosylation of TMEM165-CDG arises from a lack of Mn²⁺ within the Golgi. Nevertheless, Mn²⁺ insufficiency in the Golgi is compensated by the activity of the ATPase SERCA2. TMEM165 turnover has also been found to be regulated by Mn²⁺ cytosolic concentration. Besides causing CDG, recent investigations have demonstrated the functional involvement of TMEM165 in several other pathologies including cancer and mental health disorders. This systematic review summarizes the available information on TMEM165 molecular structure, cellular function, and its roles in health and disease.

Keywords: CDG; Golgi; Mn(2+); SERCA; SPCA1; TMEM; glycosylation; lysosomes.

Figure 1

Schematic representation of TMEM165-CDG causing mutations and proposed mechanisms of TMEM165 deficiency. Human TMEM165 protein consists of 7 transmembrane domains (TMD). CaCA2 family signature consensus sequence E-φ-G-D-(KR)-(TS) is located in TMD2 and TMD5. This motif was demonstrated to be indispensable for the Ca²⁺/Mn²⁺:H⁺ antiporter activity of TMEM165. A lysosome targeting sequence YNRL is located at TMD3. Among patients with TMEM165-CDG 5 different disease-causing mutations were identified (marked in red on the figure). Point missense mutation [p.E108G] in TMD2 targets the CaCA2 family signature consensus motif disrupting the ability of TMEM165 to bind with Ca²⁺ and Mn²⁺. Two more-point missense mutations [p.R126H] and [p.R126C] are located in the lysosome targeting motif and plausibly hampers TMEM165 translocation to Golgi. Deletion mutation (c.792 + 182G > A) in TMD5 is causing the activation of a cryptic splice donor and the drastic decrease in mRNA expression of full-length TMEM165 protein. Finally, point missense mutation [p.G304R] in TMD7 indirectly affects the conformation of the Ca²⁺/Mn²⁺ binding site.

Figure 2

Model of Golgi-ER regulation of Mn²⁺homeostasis.A, Under normal conditions, medial-Golgi is predominantly supplied with Mn²⁺ by TMEM165 and trans-Golgi by TMEM165 and SPCA1. Mn²⁺ import by SERCA2 is negligible. B, in the case of TMEM165 deficiency, medial-Golgi is deprived of Mn²⁺ halting the function of Mn²⁺-dependent glycosylation enzymes. This event is attributable to the low abundance of SPCA1 in medial-Golgi and the absence of retrograde Mn²⁺ flow from trans-Golgi to medial-Golgi. Most likely, the Mn²⁺ content in trans-Golgi is also decreased due to the absence of TMEM165 activity. Despite its ability to transport Mn²⁺, SERCA2 is not able to rescue the medial-Golgi Mn²⁺ insufficiency due to the low affinity to Mn²⁺; the high affinity of SPCA1 for Mn²⁺ eventually results in pumping all available Mn²⁺ into the trans-Golgi. C, with excessive Mn²⁺ supplementation, the SPCA1 Mn²⁺ translocation activity becomes saturated, giving the opportunity to SERCA2 to start pumping Mn²⁺ into the ER and potentially into the cis-Golgi. The anterograde flow of Mn²⁺ through the Golgi network supplies essential cofactors to the glycosylation enzymes and hence rescues the TMEM165-deficiency phenotype. D, SPCA1 deficiency results in massive Mn²⁺ accumulation in the cytosol. This phenomenon is firstly due to the loss of Mn²⁺ translocation into the Golgi lumen by SPCA1 per se. The increased concentration of cytoplasmic Mn²⁺ triggers TMEM165 degradation; thus, SPCA1 deficiency further augments the Mn²⁺ cytoplasmic level. A high concentration of Mn²⁺ allows SERCA2 to pump Mn²⁺ into the ER and subsequentially the Golgi, thereby preventing the disruption of the glycosylation processes.