674A Poster - 10. Cell biology: Cytoskeleton, organelles and trafficking
Thursday April 07, 2:00 PM - 4:00 PM

Authors:
Maren Janz ¹; Lena Dehnen ¹; Jitender Kumar Verma ²; Olympia Ekaterini Psathaki ³; Lars Langemeyer ²; Florian Fröhlich ⁴; Jürgen J. Heinisch ⁵; Heiko Meyer ¹; Christian Ungermann ²; Achim Paululat ¹

Affiliations:
1) University of Osnabrück, Department of Biology and Chemistry, Zoology and Developmental Biology; 2) University of Osnabrück, Department of Biology and Chemistry, Biochemistry; 3) University of Osnabrück, Center of Cellular Nanoanalytics, Integrated Bioimaging Facility Osnabrück (iBiOs); 4) University of Osnabrück, Department of Biology and Chemistry, Molecular Membrane Biology; 5) University of Osnabrück, Department of Biology and Chemistry, Genetics

Keywords:
l. endosomes; l. endosomes

Drosophila nephrocytes are specialized cells displaying highest endocytic activity. As a functional adaptation they display enormous cell membrane expansions and membrane tubulation where the endocytic active site is situated. The huge size of the cells also ensures enormous storage capacity, e.g. for degraded proteins or metabolic components¹. We use nephrocytes to investigate the trimeric Rab7 guanine nucleotide exchange factor (GEF) complex Mon1-Ccz1-Bulli.
During endocytosis, Rab5 decorated early endosomes mature into Rab7 positive late endosomes. Subsequently, fusion with lysosomes leads to acidification and degradation of the lysosomal content/cargo. To function on membranes, Rab proteins are activated by GEFs and further inactivated by GTPase activating proteins (GAPs). Maturation of early endosomes to late endosomes and the fusion with lysosomes also depend on the two heterohexameric tethering complexes CORVET and HOPS. The CORVET complex is an effector of Rab5 whereas HOPS interacts with activated Rab7².
In Drosophila, a trimeric Rab7 GEF complex, containing Mon1, Ccz1 and Bulli is active. We have shown that the absence of Bulli results in impaired endosomal maturation and enlarged Rab7 positive and negative endosomes with clustered Rab5 inside³. Furthermore, the lack of either Mon1 or Ccz1 reveals defective Rab5 and Rab7 localization and internalization of Rab5 into endosomes. Bulli has been identified in insects³ and mammalian cells⁴, indicating that the trimeric GEF is common in metazoan.
Overall, our data demonstrate the importance of the trimeric Rab7 GEF complex for proper localization of Rab5 and Rab7 in Drosophila. Our work aims to provide fundamental insight into the regulation of the endocytic pathway via this complex in nephrocytes. Especially the mechanism leading to Rab5 clustering in mutant flies will be investigated in future studies.

1. Weavers et al., 2009, Nature 457, 322-326.
2. Huotari & Helenius, 2011, EMBO J. 30, 3481-3500.
3. Dehnen et al., 2020, J Cell Sci 133.13.
4. Vaites et al., 2018, Mol Cell Biol 38.1.