Jos Jansen

4 95 NAFLD Phenotype in Patients With V-ATPase Proton Pump Assembly Defects transplantation was tried in two patients from the same CCDC115-deficient family. For a nine-year old girl transplantation was successful and normalization of aminotransferases and glycosylation was noted after transplantation. Unfortunately, her brother’s transplant got rejected twice and he died at the age of nine. Assembly of the V-ATPase To better understand the pathogenicity of defects in V-ATPase assembly factors, a short summary of V-ATPase physiology is necessary. For a detailed review regarding structure and function we refer the reader to sources elsewhere. (31, 32) The V-ATPase is the main proton pump of the secretory pathway. Apart from acidification of the Golgi and lysosomes, it is involved in a large number of cellular functions such as protein trafficking and membrane fissuring. The V-ATPase consists of 14 core units divided between two domains: a transmembrane V0 domain and a cytosolic V1 domain. The V1 domain provides the energy for the V0 domain to translocate protons. The V0 domain consists of a proteolipid ring (made from c, c’, and c’’ subunits) and a cage structure made from subunit a. An additional d subunit provides the link between the V0 and V1 domains (Figure 2). Little is known how the mammalian V-ATPase is constructed and regulated and most knowledge comes from yeast studies. Assembly of the V0 domain is regulated by at least four ER-resident proteins: Vph2p (yeast homolog of TMEM199) and Vma22p (homolog of CCDC115) stabilize the cage structure, Voa1p (homolog of ATP6AP1) and Vma21p (homolog of VMA21) stabilize the proteolipid ring and Vma21p chaperone the V0 domain from the ER to the Golgi. TMEM199 and CCDC115 are located in the ER-to-Golgi intermediate complex (ERGIC) and Coatomer Protein Complex 1 (COP1) vesicles in HeLa- and primary hepatoma cells.(27, 29) TMEM199, 208 amino acids small, is a transmembrane protein and shares a conserved Vma12 domain throughout species. TMEM199 was identified as a nuclear envelope transmembrane protein with a fraction distributed to endosomes in human leukocytes.(33) CCDC115 is a 181 amino acid small protein with two coiled coil domains. It is expressed mainly in the liver, heart, kidney, and testis and immunofluorescence studies located mouse Ccdc115 to endo/lysosomal structures.(34) Yeast studies confirm a transient interaction between the yeast homologs of TMEM199 and CCDC115 and the V-ATPase a-subunit.(35)