The CORVET complex promotes tethering and fusion of Rab5/Vps21-positive membranes

ArticleinProceedings of the National Academy of Sciences 110(10):3823-8 · March 2013with19 Reads
DOI: 10.1073/pnas.1221785110 · Source: PubMed
Abstract
Membrane fusion along the endocytic pathway occurs in a sequence of tethering, docking, and fusion. At endosomes and vacuoles, the CORVET (class C core vacuole/endosome tethering) and HOPS (homotypic fusion and vacuole protein sorting) tethering complexes require their organelle-specific Rabs for localization and function. Until now, despite the absence of experimental evidence, it has been assumed that CORVET is a membrane-tethering factor. To test this theory and understand the mechanistic analogies with the HOPS complex, we set up an in vitro system, and establish CORVET as a bona-fide tether for Vps21-positive endosome/vacuole membranes. Purified CORVET binds to SNAREs and Rab5/Vps21-GTP. We then demonstrate that purified CORVET can specifically tether Vps21-positive membranes. Tethering via CORVET is dose-dependent, stimulated by the GEF Vps9, and inhibited by Msb3, the Vps21-GAP. Moreover, CORVET supports fusion of isolated membranes containing Vps21. In agreement with its role as a tether, overexpressed CORVET drives Vps21, but not the HOPS-specific Ypt7 into contact sites between vacuoles, which likely represent vacuole-associated endosomes. We therefore conclude that CORVET is a tethering complex that promotes fusion of Rab5-positive membranes and thus facilitates receptor down-regulation and recycling at the late endosome.
    • "Finally, for many of these complexes, the individual interactions were identified but not the function of these interactions in trafficking. CORVET interacts with endosomal/vacuolar SNARE proteins and SNARE complexes through the Vps33 subunit (Subramanian et al., 2004; Balderhaar et al., 2013). The Dsl1 complex interacts with t-SNARE proteins on the ER, potentially in lieu of interactions with small GTPases, and its mammalian counterpart NZR interacts with various ER-localized SNARE proteins (Kraynack et al., 2005; Meiringer et al., 2011; Tagaya et al., 2014). "
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    • "The transition from PGEs to EEs is mediated by RAB5s (Baschong and Riezman, 2002). The transition of EEs into LEs is determined by the substitution of RAB5 by RAB7 (RabB RAB5 /Vps21 and RabS RAB7 /Ypt7 in A. nidulans and S. cerevisiae, respectively) (Rink et al., 2005; Peplowska et al., 2007; Markgraf et al., 2009; Abenza et al., 2010; Nordmann et al., 2010; Poteryaev et al., 2010; Epp et al., 2011; Balderhaar et al., 2013; Kummel and Ungermann, 2014). LEs undergo fusion among them and with vacuoles, in a process that mediated by HOPS, a RabS RAB7 /Ypt7 effector. "
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    • "In comparison with the function of HOPS, that of CORVET is less well understood. CORVET functions upstream of HOPS, probably in early endosomes as it binds to GTP-Rab5, and may tether vesicles or act as a tether to promote homotypic endosome fusion, a process necessary for the generation of multivesicular bodies [16,139]. CORVET appears to act independently of the coiled-coil early endosome tether EEA1 (Vac1) [140]. "
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