A trimeric metazoan Rab7 GEF complex is crucial for endocytosis and scavenger function

Abstract

Endosome biogenesis in eukaryotic cells is critical for nutrient uptake and plasma membrane integrity. Early endosomes initially contain Rab5, which is replaced by Rab7 on late endosomes prior to their fusion with lysosomes. Recruitment of Rab7 to endosomes requires the Mon1-Ccz1 guanosine exchange factor (GEF). Here, we show that full function of the Drosophila Mon1-Ccz1 complex requires a third stoichiometric subunit, termed Bulli. Bulli localises to Rab7 positive endosomes, in agreement with its function in the GEF complex. Using Drosophila nephrocytes as a model system, we observe that absence of Bulli results in (i) reduced endocytosis, (ii) Rab5 accumulation within non-acidified enlarged endosomes, and (iii) defective Rab7 localisation and (iv) impaired endosomal maturation. Moreover, longevity of animals lacking bulli is affected. Both Mon1-Ccz1 dimer and a Bulli-containing trimer display Rab7 GEF activity. In summary, this suggests a key role of Bulli in Rab5 to Rab7 transition during endosomal maturation rather than a direct influence on the GEF activity of Mon1-Ccz1.