I owe special thanks to Marnix Wieffer and Michael Krauß, who spent a lot of time helping me to get started in the lab, to overcome numerous experimental problem, I was not sure how to deal with, and frequently joined in discussions about my project with excellent advice. I am really grateful for your experimental support, Michael, and the time you invested in the project during our paper revision.Further, I would like to thank Jocelyn Laporte and his group, especially Anne-Sophie Nicot, at the IGBMC in Strasbourg for their support and a very fruitful collaboration, and Carsten Schultz and his group at the EMBL in Heidelberg for their support and constant supply with PIP/AMs. I owe special thanks to Dmytro Puchkov for the ultrastructural analysis and Eberhard Krause for mass spectrometry done within this project. I also need to acknowledge Markus Wenk and Federico Torta at the Singapore Lipidomics Incubator for joining the project at a very late stage, when we urgently needed help from lipidomics specialists. It was a pity that experiments did not work out as planned.I would further like to express my gratitude to two very skilled technicians in the AG Haucke lab: Silke Zillmann and Delia Löwe. Without your help it would have been impossible to achieve so much within the limited amount of time I had during the paper revision. by VPS34-IN1 treatment................................................... 52 2.3.11 Fluorescence microscopy................................................................................. 53 2.3.12 Flow cytometry............................................................................................
III SummaryPhosphoinositides (PIs) are a minor class of short-lived phospholipids that serve as crucial signposts of membrane identity. Thereby, PIs full fill important functions in cell signaling and membrane transport. PI 4-phosphates such as phosphatitylinositol-4-phosphate (PI(4)P) and phosphatitylinositol-4,5-bisphosphate (PI(4,5)P 2 ) are enriched at the plasma membrane (PM), on secretory organelles and lysosomes, while PI 3-phosphates, i.e.phosphatitylinositol-3-phosphate (PI(3)P), are a hallmark of the endosomal system.Directional transport between these compartments, thus, requires regulated PI conversion.However, PI conversion in exit from PI(3)P-enriched endosomes en route to the PI(4)P-and PI(4,5)P 2 -positive PM in endosomal recycling remained unknown.Here, we report that cargo exit from endosomes requires removal of PI(3)P by the PI(3)P 3-phosphatase myotubularin 1 (MTM1), and concomitant PI(4)P synthesis by PI 4-kinase type II α (PI4K2α). Loss of MTM1 causes endosomal accumulation of PI(3)P and PI(3)P effector proteins, i.e. sorting nexins, Kif16b-mediated outward traffic of PI(3)P containing endosomes and sub-plasmalemmal accumulation of exocytosis-deficient endosomes. As PI4K2α associates with MTM1 and thereby facilitates membrane recruitment of MTM1, these phenotypic changes are mimicked by loss of PI4K2α. The conversion of PI(3)P-to-PI(4)P is paralleled by a switch i...