VMA12

VMA12 (yeast Vma12p/Vph2p; human TMEM199) is an integral endoplasmic reticulum membrane protein that acts as a dedicated assembly factor for the vacuolar/lysosomal H+-ATPase (V-ATPase) rather than as a structural subunit of the active enzyme (PMID:8419376, PMID:9325326). In yeast lacking Vma12p, peripheral V1 subunits fail to localize to the vacuolar membrane and the membrane-embedded V0 subunit is rapidly degraded after its insertion into the ER, demonstrating that Vma12p promotes assembly and stabilization of V0 in the ER and is required for vacuolar proton pumping and ATPase activity (PMID:8419376, PMID:8465604, PMID:9325326). This function is conserved in the human homolog: loss-of-function mutations in TMEM199 cause a congenital disorder of glycosylation, with deficient Golgi N- and O-glycosylation that is restored by re-expression of wild-type TMEM199, and the protein localizes to the ERGIC/COPI compartment (PMID:26833330). Through its control of V-ATPase-dependent endo-lysosomal acidification, TMEM199 governs cellular iron homeostasis, such that its loss depletes intracellular iron, reduces prolyl hydroxylase activity, and stabilizes HIF1α, while iron supplementation restores PHD activity (PMID:28296633). TMEM199 is similarly required for lysosomal acidification and lipophagy, and its deficiency drives lipid droplet accumulation and hepatic steatosis (PMID:34626841). Together with its partner CCDC115, TMEM199 also interacts with the IFN-γ receptors IFNGR1/2 and recruits the TRAPP II complex to activate RAB11A, promoting receptor recycling, JAK-STAT signaling, and PD-L1 expression (PMID:41319859).

1. 1993 High

   Established that Vma12p is needed to build the V-ATPase but is not itself part of the enzyme, defining it as an assembly/targeting factor rather than a structural subunit.

   Evidence Chromosomal null mutant with Western blotting of vacuolar membrane fractions and glycerol-gradient purification of the V-ATPase complex in S. cerevisiae

   PMID:8419376 PMID:8465604

   Open questions at the time

   • Did not localize where in the cell Vma12p acts
   • Did not define the molecular step of assembly it catalyzes
2. 1997 High

   Localized Vma12p function to the ER and showed it stabilizes the V0 subunit after membrane insertion, pinpointing the assembly step as ER V0 maturation.

   Evidence Protease protection, membrane fractionation, immunolocalization, and pulse-chase degradation assays in vma12Δ yeast

   PMID:9325326

   Open questions at the time

   • No direct V0 subunit-binding interaction defined
   • Whether other subunits depend on Vma12p for ER stability not resolved
3. 2016 High

   Connected the human homolog TMEM199 to disease, showing its loss causes a glycosylation disorder via disrupted Golgi homeostasis.

   Evidence Exome sequencing of CDG patients, glycan mass spectrometry, lentiviral rescue, and ERGIC/COPI co-localization in HeLa cells

   PMID:26833330

   Open questions at the time

   • Did not directly demonstrate V-ATPase assembly defect in human cells
   • Link between acidification and glycosylation not fully mechanistic in this study
4. 2017 High

   Defined a mechanistic chain from TMEM199 to HIF1α by showing V-ATPase-dependent acidification controls iron homeostasis and thereby PHD activity.

   Evidence Genome-wide haploid genetic screen, TMEM199/CCDC115 disruption, HIF1α stabilization assays, and iron supplementation rescue of PHD activity

   PMID:28296633

   Open questions at the time

   • Mechanism by which V-ATPase loss depletes iron not fully detailed
   • Did not address tissue-specific consequences
5. 2018 Medium

   Extended the ER-localized V-ATPase assembly role to a pathogenic fungal ortholog, linking it to vacuolar acidification, endocytosis, cell wall integrity, and virulence.

   Evidence Deletion mutants, V-ATPase subunit localization microscopy, acidification/endocytosis/CWI assays, and a candidiasis mouse model in C. albicans

   PMID:29522815

   Open questions at the time

   • Single lab in a fungal ortholog
   • Direct subunit interactions not mapped
6. 2019 Medium

   Implicated Vph2 in ER redox balance by showing its loss elevates GSH and induces the UPR, broadening its role beyond V-ATPase assembly.

   Evidence ER localization microscopy, reductive stress sensitivity assays, GSH quantification, UPR gene expression, and CDNB chemical rescue in C. albicans

   PMID:30928095

   Open questions at the time

   • Causal connection between V-ATPase function and GSH levels unresolved
   • Single lab, fungal ortholog
7. 2021 Medium

   Showed that loss of TMEM199-dependent acidification blocks lipophagy and drives hepatic steatosis, providing a physiological consequence of impaired lysosomal function.

   Evidence siRNA knockdown in HepG2, CRISPR knock-in mouse, patient iPSC-derived hepatocytes, lipid droplet/lysosome imaging, and autophagic flux assays

   PMID:34626841

   Open questions at the time

   • Lipophagy mechanism established by a single lab
   • Molecular link from acidification defect to lipid droplet retention not fully detailed
8. 2025 Medium

   Revealed a trafficking function in which TMEM199/CCDC115 recruits TRAPP II to activate RAB11A and recycle IFN-γ receptors, coupling the protein to immune signaling and PD-L1 expression.

   Evidence Co-immunoprecipitation with IFNGR1/2, trafficking microscopy to RAB11A endosomes, TRAPP II recruitment and RAB11A activation assays, and PD-L1 expression readouts

   PMID:41319859

   Open questions at the time

   • Single Co-IP-based study not yet independently replicated
   • Whether this function is independent of V-ATPase assembly role unclear
   • Direct vs. indirect IFNGR1/2 interaction not resolved

• How TMEM199 physically engages V0 subunits and whether its endosomal trafficking/RAB11A role is mechanistically separable from its canonical V-ATPase assembly function remain unresolved.
• No structural model of TMEM199 bound to V-ATPase subunits
• Direct substrate/subunit contacts unmapped
• Relationship between assembly-factor and trafficking roles undefined