Department of Chemistry and Pharmacy, Institute of Biochemistry, University of Münster, 48149 Münster, Germany.
Department of Biology/Chemistry, Structural Biology section, Osnabrück University, 49076 Osnabrück, Germany.
Proc Natl Acad Sci U S A. 2023 May 16;120(20):e2301908120. doi: 10.1073/pnas.2301908120. Epub 2023 May 8.
The endosomal system of eukaryotic cells represents a central sorting and recycling compartment linked to metabolic signaling and the regulation of cell growth. Tightly controlled activation of Rab GTPases is required to establish the different domains of endosomes and lysosomes. In metazoans, Rab7 controls endosomal maturation, autophagy, and lysosomal function. It is activated by the guanine nucleotide exchange factor (GEF) complex Mon1-Ccz1-Bulli (MCBulli) of the tri-longin domain (TLD) family. While the Mon1 and Ccz1 subunits have been shown to constitute the active site of the complex, the role of Bulli remains elusive. We here present the cryo-electron microscopy (cryo-EM) structure of MCBulli at 3.2 Å resolution. Bulli associates as a leg-like extension at the periphery of the Mon1 and Ccz1 heterodimers, consistent with earlier reports that Bulli does not impact the activity of the complex or the interactions with recruiter and substrate GTPases. While MCBulli shows structural homology to the related ciliogenesis and planar cell polarity effector (Fuzzy-Inturned-Wdpcp) complex, the interaction of the TLD core subunits Mon1-Ccz1 and Fuzzy-Inturned with Bulli and Wdpcp, respectively, is remarkably different. The variations in the overall architecture suggest divergent functions of the Bulli and Wdpcp subunits. Based on our structural analysis, Bulli likely serves as a recruitment platform for additional regulators of endolysosomal trafficking to sites of Rab7 activation.
真核细胞的内体系统是一个中央分拣和回收隔室，与代谢信号和细胞生长的调节有关。需要严格控制 Rab GTPases 的激活，以建立内体和溶酶体的不同结构域。在后生动物中，Rab7 控制着内体成熟、自噬和溶酶体功能。它被三长同源结构域（TLD）家族的鸟嘌呤核苷酸交换因子（GEF）复合物 Mon1-Ccz1-Bulli（MCBulli）激活。虽然已经表明 Mon1 和 Ccz1 亚基构成了复合物的活性位点，但 Bulli 的作用仍不清楚。我们在此展示了 3.2Å分辨率的 MCBulli 的冷冻电镜（cryo-EM）结构。Bulli 作为一个类似于腿的延伸部分与 Mon1 和 Ccz1 异二聚体的外围相关联，这与早期的报道一致，即 Bulli 不会影响复合物的活性或与招募物和底物 GTPases 的相互作用。虽然 MCBulli 显示出与相关的纤毛发生和平板细胞极性效应物（Fuzzy-Inturned-Wdpcp）复合物的结构同源性，但 TLD 核心亚基 Mon1-Ccz1 与 Fuzzy-Inturned 分别与 Bulli 和 Wdpcp 的相互作用非常不同。总体结构的变化表明 Bulli 和 Wdpcp 亚基的功能存在差异。基于我们的结构分析，Bulli 可能作为内体溶酶体运输的额外调节剂到 Rab7 激活部位的招募平台。
