The GTPase-activating protein GRAF1 regulates the CLIC/GEEC endocytic pathway

Medical Research Council, Laboratory of Molecular Biology, Hills Road, Cambridge, CB2 0QH, UK.
Current Biology (Impact Factor: 9.92). 12/2008; 18(22):1802-8. DOI: 10.1016/j.cub.2008.10.044
Source: PubMed

ABSTRACT Clathrin-independent endocytosis is an umbrella term for a variety of endocytic pathways that internalize numerous cargoes independently of the canonical coat protein Clathrin [1, 2]. Electron-microscopy studies have defined the pleiomorphic CLathrin-Independent Carriers (CLICs) and GPI-Enriched Endocytic Compartments (GEECs) as related major players in such uptake [3, 4]. This CLIC/GEEC pathway relies upon cellular signaling and activation through small G proteins, but mechanistic insight into the biogenesis of its tubular and tubulovesicular carriers is lacking. Here we show that the Rho-GAP-domain-containing protein GRAF1 marks, and is indispensable for, a major Clathrin-independent endocytic pathway. This pathway is characterized by its ability to internalize bacterial exotoxins, GPI-linked proteins, and extracellular fluid. We show that GRAF1 localizes to PtdIns(4,5)P2-enriched, tubular, and punctate lipid structures via N-terminal BAR and PH domains. These membrane carriers are relatively devoid of caveolin1 and flotillin1 but are associated with activity of the small G protein Cdc42. This study provides the first specific noncargo marker for CLIC/GEEC endocytic membranes and demonstrates how GRAF1 can coordinate small G protein signaling and membrane remodeling to facilitate internalization of CLIC/GEEC pathway cargoes.

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Available from: Katia Cortese, Aug 31, 2015
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    • "Nevertheless, these arguments all highlight the complexities in studying vesicular uptake. It is also worth mentioning that there are several other clathrin-independent endocytic pathways, such as the recently described CLIC/GLEEC pathway (78, 152). The extent to which these other pathways may be involved remains to be determined. "
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    ABSTRACT: Extracellular vesicles (EVs) are small vesicles released by donor cells that can be taken up by recipient cells. Despite their discovery decades ago, it has only recently become apparent that EVs play an important role in cell-to-cell communication. EVs can carry a range of nucleic acids and proteins which can have a significant impact on the phenotype of the recipient. For this phenotypic effect to occur, EVs need to fuse with target cell membranes, either directly with the plasma membrane or with the endosomal membrane after endocytic uptake. EVs are of therapeutic interest because they are deregulated in diseases such as cancer and they could be harnessed to deliver drugs to target cells. It is therefore important to understand the molecular mechanisms by which EVs are taken up into cells. This comprehensive review summarizes current knowledge of EV uptake mechanisms. Cells appear to take up EVs by a variety of endocytic pathways, including clathrin-dependent endocytosis, and clathrin-independent pathways such as caveolin-mediated uptake, macropinocytosis, phagocytosis, and lipid raft–mediated internalization. Indeed, it seems likely that a heterogeneous population of EVs may gain entry into a cell via more than one route. The uptake mechanism used by a given EV may depend on proteins and glycoproteins found on the surface of both the vesicle and the target cell. Further research is needed to understand the precise rules that underpin EV entry into cells.
    08/2014; 3:24641. DOI:10.3402/jev.v3.24641
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    • "The activity of Arf1 itself appears to be regulated by a GEF called GBF1 at the cell surface [8]. The Bar-domain protein GRAF1 participates in the post-endocytic dynamics of the GEECs [15]. It is clear that a host of other core and peripheral molecules must drive the trafficking of these compartments and their cargo to specific destinations inside the cell. "
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    PLoS ONE 06/2014; 9(6):e100554. DOI:10.1371/journal.pone.0100554 · 3.23 Impact Factor
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    • "The bulk uptake of plasma membrane and extracellular fluid, however, is predominantly through Clathrin-independent endocytic (CIE) mechanisms, which account for an estimated 40–70% of constitutive fluid-phase internalization depending on the cell type (Cheng et al., 2006; Howes et al., 2010; Sabharanjak et al., 2002; Sandvig et al., 1987). Whether Dynamin regulates CIE mechanisms and, in particular those that involve the uptake of fluid-phase, is a debated issue as different means of Dynamin inhibition have varying effects depending on the experimental context (Altschuler et al., 1998; Cao et al., 2007; Damke et al., 1994, 1995; Guha et al., 2003; Herskovits et al., 1993; Kosaka and Ikeda, 1983b; Lundmark et al., 2008; Macia et al., 2006; Sabharanjak et al., 2002). "
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