2.A.106 Ca²⁺:H⁺ Antiporter-2 (CaCA2) Family

The PF27 family (SwissProt family UPF0016; Prosite entry PDOC00934) is a small family of proteins found in bacteria, archaea, yeast, plants and animals. They are usually 200-350 amino acyl residues long and exhibit 5-7 TMSs. Congenital Disorders of Glycosylation (CDG) are rare inherited diseases causing glycosylation defects responsible for severe growth and psychomotor retardations in patients. TMEM165 has been implicated in both Golgi vesicular trafficking and ion homeostasis. TMEM165 deficient mammalian cells or yeast cells deficient in Gtd1p, the yeast TMEM165 ortholog suggest that TMEM165 is a transporter involved in ion homeostasis (Dulary et al. 2017). Several studies indicate that TMEM165 is a Golgi localized Ca²⁺/H⁺ and/or Mn²⁺/H⁺ antiporter. Possibly TMEM165 proteins are Mn²⁺ transporters essential to achieve the correct N-glycosylation process of proteins in the secretory pathway.

Defects in the human TMEM165 homologue are the cause of congenital disorder of glycosylation type 2K (CDG2K), an autosomal recessive disorder with variable phenotypes (Foulquier et al.2012). Affected individuals show psychomotor and growth retardation, and most have short stature. Other features include dysmorphism, hypotonia, eye abnormalities, acquired microcephaly, hepatomegaly, and skeletal dysplasia. Congenital disorders of glycosylation are caused by a defect in glycoprotein biosynthesis and are characterized by under-glycosylated serum glycoproteins and a wide variety of clinical features. The broad spectrum of features may reflect the critical role of N-glycoproteins during embryonic development, differentiation, and maintenance of cell functions (Foulquier et al. 2012).

The Ca²⁺:H⁺  Antiporter (CaCA2) Family (previous called the uncharacterized Protein Family 0016 (UPF0016), is well conserved throughout prokaryotes and eukaryotes. The yeast golgi Gcr1-dependent translation factor 1 protein (Gdt1p) contributes to Ca²⁺ homeostasis. A yeast gdt1 mutant was found to be sensitive to high concentrations of Ca²⁺, and this sensitivity was suppressed by expression of human TMEM165 in yeast (Demaegd et al.2013). Patch-clamp analyses on human cells indicated that TMEM165 catalyzes Ca²⁺ transport, and defects in TMEM165 affected both Ca²⁺ and pH homeostasis.Gdt1p and TMEM165 are probably Golgi-localized Ca²⁺:H⁺  antiporters. Modification of the Golgi Ca²⁺ and pH balance could explain the glycosylation defects observed in TMEM165-deficient patients. 

The UPF0016 family is defined by the presence of 1 or 2 copies of the E-phi-G-D-[KR]-[TS] consensus motif in their transmembrane domain. Colinet et al. 2017 showed that 2 members of this family, the human TMEM165 and the budding yeast Gdt1p, are functionally related Ca²⁺ transporters. Mutations in TMEM165 cause a type of rare human genetic diseases called Congenital Disorders of Glycosylation. Mutations in the yeast Golgi-localized Ca²⁺ transporter, Gdt1, revealed that acidic and polar uncharged residues of the consensus motifs are involved in calcium tolerance and calcium transport activity. The E53 residue corresponds to the mutation in humans that triggers congenital disorders of glycosylation (Colinet et al. 2017).

The generalized reaction catalyzed by CaCA2 family members is:

Ca²⁺ (cytoplasm) + H⁺ (golgi lumen) → Ca²⁺ (golgi lumen) + H⁺ (cytoplasm)