期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：6
DOI：10.1073/pnas.2121494119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Rab GTPases are central regulators of intracellular trafficking and serve as markers of organelle identity. They act as molecular switches, and their activation requires precise spatiotemporal control. Members of the family of the Tri Longin domain (TLD) Rab-GEFs (guanine nucleotide exchange factors) act as activators of a subset of Rabs that play a critical role in late endosomal biogenesis. Genetic defects associated with TLD Rab-GEFs cause developmental diseases, but the underlying mechanisms are only partly understood. The determination of the structure of the TLD Rab-GEF Mon1-Ccz1 presented here provides a molecular basis for understanding the function and regulation of these proteins.
Activation of the GTPase Rab7/Ypt7 by its cognate guanine nucleotide exchange factor (GEF) Mon1-Ccz1 marks organelles such as endosomes and autophagosomes for fusion with lysosomes/vacuoles and degradation of their content. Here, we present a high-resolution cryogenic electron microscopy structure of the Mon1-Ccz1 complex that reveals its architecture in atomic detail. Mon1 and Ccz1 are arranged side by side in a pseudo-twofold symmetrical heterodimer. The three Longin domains of each Mon1 and Ccz1 are triangularly arranged, providing a strong scaffold for the catalytic center of the GEF. At the opposite side of the Ypt7-binding site, a positively charged and relatively flat patch stretches the Longin domains 2/3 of Mon1 and functions as a phosphatidylinositol phosphate–binding site, explaining how the GEF is targeted to membranes. Our work provides molecular insight into the mechanisms of endosomal Rab activation and serves as a blueprint for understanding the function of members of the Tri Longin domain Rab-GEF family.
