Jos Jansen

96 Chapter 4 ATP6AP1 is located on the X chromosome and encodes the 470 amino acid long Ac45 protein. Ac45 is widely accepted as a V-ATPase accessory protein and has been studied extensively in Xenopus laevia (frog) hormone-secreting cells and mouse osteoclasts. 20,21 Ac45 is cleaved by furin in two fragments in the early secretory pathway.(36, 37) After cleavage, the cytosolic C-terminal fragment traffics to secretory vesicles, along with the V-ATPase.(38) Recent studies demonstrate that the C-terminal fragment of human Ac45 shares homology with Voa1p, a yeast V-ATPase assembly factor.(28) Ac45 is located in the ER and ERGIC and probably interacts with the c’’ subunit of the proteolipid ring in yeast and mouse osteoclasts.(39, 40) In frog melanotrope cells, overexpression resulted in lower pH in granules and subsequent cell-specific effects, possible due to increased secretion efficiency.(39, 41, 42) Knockdown of Ac45 in mouse osteoclasts resulted in increased pH, impaired lysosomal trafficking, and lysosomal exocytosis.(43) Mass spectrometric analysis of serum transferrin derived from TMEM199, CCDC115, and ATP6AP1 deficient patients show abnormal N-glycosylation with loss of sialic acids and galactose from glycans. Additionally, isoelectric focusing of Apolipoprotein CIII demonstrated loss of sialic acids, indicative for abnormal O-glycosylation. VMA21 is the human homolog of yeast Vma21p. VMA21 deficiency (MIM: 310440) is the cause of XMEA (Myopathy, X-linked, with Excessive Autophagy) a disorder dominated by a neuromuscular phenotype. In yeast, Vma21p escorts the V0 complex out of the ER in COPII vesicles. After delivering the V0 domain to the Golgi, Vma21p dissociates and is retrieved to the ER by an ER retention motif, which is absent in mammalian VMA21.(44) Interestingly, Vma21p was implicated in abnormal lipid homeostasis.(45, 46) Human VMA21 locates to the ER-to-Golgi region and missense mutations lead to decreased formation of V0 complexes and increased numbers of autolysosomes, characteristic for the XMEA phenotype.(47) Proposed cellular model of V-ATPase assembly factors Based on the data mentioned above we propose a model were these four proteins aid in the assembly of the V-ATPase V0 domain in humans. CCDC115 and TMEM199 stabilize the a-subunit during assembly and VMA21 and cleaved