Myotubularins are lipid phosphatases that dephosphorylate phosphatidylinositol 3-phosphate and phosphatidylinositol 3,5-bisphosphate the position three of the inositol ring. This allows them to regulate the structure of the lipid layer of the membrane compartment. The first member of the family was described in association with a severe hereditary myopathy. From that point on, another thirteen members have been added to the family. The catalytically inactive MTMR9 carrying the conserved mutation in the phosphatase domain regulates the localization of the marker of the early secretory pathway, RAB1A, the cis-Golgi structure and the secretion. MTMR9 interacts with the catalytically active MTMR6 and MTMR8 that specifically localizes and increases their phophatase activity. The aim of this diploma thesis was to find out whether the phenotype observed in cells with altered MTMR9 levels is dependent on the catalytically active phosphatases MTMR6 and MTMR8. We proved the influence of MTMR6 and MTMR8 on the distribution of tranfected RAB1A between the intermediate compartment and the Golgi apparatus. MTMR6 and MTMR8 also take part in regulating the cis-Golgi structure. By the use of two different approaches we did not manage to clarify the influence of MTMR6 and MTMR8 on secretion. Changes in the catalytic...