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 guanine-nucleotide-exchange factor (GEF). Here, we show that full function of the Drosophila Mon1–Ccz1 complex requires a third stoichiometric subunit, termed Bulli (encoded by CG8270). 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, (iii) defective Rab7 localisation and (iv) impaired endosomal maturation. Moreover, longevity of animals lacking bulli is affected. Both the Mon1–Ccz1 dimer and a Bulli-containing trimer display Rab7 GEF activity. In summary, this suggests a key role for Bulli in the Rab5 to Rab7 transition during endosomal maturation rather than a direct influence on the GEF activity of Mon1–Ccz1.

真核细胞中的内体生物发生对于营养吸收和质膜完整性至关重要。早期内涵体最初含有 Rab5，晚期内涵体在与溶酶体融合之前被 Rab7 取代。Rab7 招募到内体需要 Mon1–Ccz1 鸟嘌呤核苷酸交换因子 (GEF)。在这里，我们表明果蝇Mon1-Ccz1 复合物的完整功能需要第三个化学计量亚基，称为 Bulli（由CG8270编码）。Bulli 定位于 Rab7 阳性内体，与其在 GEF 复合物中的功能一致。使用果蝇肾细胞作为模型系统，我们观察到 Bulli 的缺失会导致 (i) 内吞作用减少，(ii) Rab5 在非酸化扩大的内体中积累，(iii) Rab7 定位缺陷和 (iv) 内体成熟受损。此外，缺乏大牛的动物的寿命也会受到影响。Mon1–Ccz1 二聚体和包含 Bulli 的三聚体均显示 Rab7 GEF 活性。总之，这表明 Bulli 在内体成熟过程中 Rab5 到 Rab7 转变中发挥关键作用，而不是直接影响 Mon1-Ccz1 的 GEF 活性。