Rubicon controls endosome maturation as a Rab7 effector

Division of Biochemistry and Molecular Biology, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 10/2010; 107(45):19338-43. DOI: 10.1073/pnas.1010554107
Source: PubMed


The activation and recruitment of the small GTPase Rab7 to early endosome is a critical step for early to late endosome maturation, a process that requires the class III phosphatidylinositol 3-kinase (PI3KC3) and GTPase regulators. However, the molecular mechanism underlying Rab7 activation and endosome maturation is still poorly defined. Here we report that Rubicon, a component of the PI3KC3 complex, prevents endosome maturation through differential interactions with Rab7 and UVRAG. UVRAG activates PI3KC3 and C-VPS/HOPS, a guanine nucleotide exchange factor that catalyzes the exchange of GDP for GTP on Rab7. We demonstrate that Rubicon sequesters UVRAG from C-VPS/HOPS. Active GTP-bound Rab7 competes for Rubicon binding and releases UVRAG to associate with C-VPS/HOPS, which in turn promotes further loading of Rab7 with GTP. This feed-forward loop ensures rapid amplification of GTP-bound Rab7 and consequent stimulation of endosome maturation. Hence, Rubicon serves as a previously unknown Rab7 effector to ensure the proper progression of the endocytic pathway.

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    • "RESULTS Arl8b, but not Rab7, interacts with the hVps41 subunit of the HOPS complex in a GTP-dependent manner In our previous study, we identified hVps41 as a direct binding partner for Arl8b, which is recruited to lysosomes in an Arl8b- dependent manner (Garg et al., 2011). However, several reports have implicated a role for Rab7 and/or its effector RILP in regulating late endosomal or lysosomal recruitment of the HOPS complex through interaction with the Vps41 subunit (Sun et al., 2010; van der Kant et al., 2013; Lin et al., 2014). "
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    ABSTRACT: HOmotypic fusion and Protein Sorting (HOPS) complex is a multi-subunit complex conserved from yeast to mammals that regulates late endosome-lysosome fusion. However, little is known about how HOPS complex is recruited to lysosomes in mammalian cells. Here we report that the small GTPase Arl8b, but not Rab7, is essential for membrane localization of hVps41 subunit of the HOPS complex. Assembly of the core HOPS subunits to Arl8b and hVps41-positive lysosomes is guided by their subunit-subunit interactions. RNAi-mediated depletion of hVps41 resulted in the impaired degradation of EGFR that was rescued upon expression of wild-type but not an Arl8b-binding defective mutant of hVps41, suggesting that Arl8b-dependent lysosomal localization of hVps41 is required for its endocytic function. Further, we have also identified that Arl8b effector SKIP/PLEKHM2 interacts with and recruits HOPS subunits to Arl8b and Kinesin-positive peripheral lysosomes. Accordingly, RNAi-mediated depletion of SKIP impaired lysosomal trafficking and degradation of EGFR. These findings reveal that Arl8b regulates association of the human HOPS complex with lysosomal membranes that is critical for the function of this tethering complex in endocytic degradation. © 2015. Published by The Company of Biologists Ltd.
    Journal of Cell Science 04/2015; 128(9). DOI:10.1242/jcs.162651 · 5.43 Impact Factor
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    • "While the Ambra1-containing complex is required for the induction of autophagy [24], Atg14L/Barkor is thought to function in recruiting the Vps34-Vps15-Atg6 complex to the autophagosomal membrane to initiate autophagosome formation [25]. Rubicon, in contrast, negatively regulates autophagy and endocytosis by preventing the activation of Rab7, a protein that functions in lysosomal fusion and autophagosome maturation [22] [26] [27]. The role of UVRAG in autophagy remains controversial. "
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    ABSTRACT: The interconnection of the endocytic and autophagosomal trafficking routes has been recognized more than two decades ago with both pathways using a set of identical effector proteins and sharing the same ultimate lysosomal destination. More recent data sheds light onto how other pathways are intertwined into this network, and how degradation via the endosomal/autophagosomal system may affect signaling pathways in multicellular organisms. Here, we briefly review the common features of autophagy and endocytosis and discuss how other players enter this mix with particular respect to the Notch signaling pathway.
    BioMed Research International 04/2014; 2014:960803. DOI:10.1155/2014/960803 · 2.71 Impact Factor
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    • "In support of this view, we found that disruption of the C/VPS complex binding site prevents UVRAG from Rab7 activation and inducing endosome fusion (Liang et al., 2008). This view is further reinforced in the work by Sun et al. (2010), which showed that UVRAG acts in concert with Rab7 and the negative regulator Rubicon to balance the endocytic vesicles' flux. It appears that the Rubicon protein sequesters UVRAG from the C/VPS complex interaction, wherease activated Rab7 releases UVRAG from Rubicon inhibition and promotes the UVRAG–C/VPS complex assembly, which in turn activates more Rab7 for endosome maturation. "
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    ABSTRACT: Functional inactivation of UVRAG is implicated in common human cancers. It was originally accepted that UVRAG functions as a promoter of the autophagy pathway by forming a stable complex with Beclin 1. However, recent studies have highlighted novel non-autophagic roles of UVRAG, and these reports indi- cate that UVRAG is much more versatile than we originally expected. In addition to regulating catabolic autophagy, UVRAG is also involved in regulating endosomal membrane trafficking, repairing DNA lesions, patrolling the centrosome, and preventing apoptosis. These studies are beginning to shed light on the com- plexity and cross-talk between the signaling networks involving UVRAG, which normally control diverse cellular processes, and how disruption of these networks leads to tumor formation.
    Edited by M. A. Hayat, 09/2013; Elsevier., ISBN: 9780124115149
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