Jos Jansen

4 97 NAFLD Phenotype in Patients With V-ATPase Proton Pump Assembly Defects Ac45 stabilize the c-ring and guide the V0 domain (or the holocomplex with the V1 domain) to the Golgi. VMA21, and possibly CCDC115 and TMEM199 are then transported back via COPI vesicles (Figure 2). Of note, only VMA21 has been proven to be a bonafide assembly factor in humans. The experimental evidence that assigns a role of TMEM199, CCDC115, and Ac45 as human V-ATPase assembly factors still needs to be generated. There are several lines of evidence that do support this hypothesis. First, these proteins are reciprocal best hits of their yeast homolog in BLAST searches. Second, TMEM199, CCDC115, and Ac45 (and additional V-ATPase associated proteins) are interacting partners established in high throughput protein- protein interaction studies in drosophila as well as in human. ((48) and in the Biogrid database (www.thebiogrid.org )). Third, all proteins share the ER-to-Golgi region as location and lastly, patients with defects in either TMEM199, CCDC115, and ATP6AP1 have a very similar phenotype in common. Defects in V-ATPase assembly factors lead to abnormal glycosylation because they impact ER-to-Golgi protein trafficking. Indeed, V-ATPase assembly factors co-localize with COPI, COPII, and ERGIC markers indicating a function in early protein transport. In addition, serum from patients with a –V-ATPase assembly factors have a specific glycosylation pattern, indicative for abnormal Golgi protein trafficking. Disturbed protein trafficking in the ER-to-Golgi region in NAFLD pathogenesis Hepatic steatosis is common to all identified V-ATPase assembly factor deficiencies. Lipid droplets (LDs) are the main cellular storage compartment of TGs and play an important role in steatogenesis.(49, 50) Their role as a dynamic organelle has been established by identification of multiple LD associated proteins (LDAPs), present on the LD surface.(51-53) For a comprehensive review on lipid droplets we refer the reader elsewhere.(53) Because of the emerging role of COPI complex in hepatic lipid metabolism and the colocalization of CCDC115 and TMEM199 (and possibly ATP6AP1) in COPI vesicles we focus on the effect of COPI on LDAPs trafficking and the Sterol Regulatory Element-Binding Proteins (SREBPs) pathway. Additionally, VLDL transport from the ER to the Golgi occurs through COPII vesicles and is also dependent on accurate ER-to-Golgi trafficking.