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: 9.57). 12/2008; 18(22):1802-8. DOI: 10.1016/j.cub.2008.10.044
Source: PubMed


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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    • "CLDN18 protein was observed in the plasma membrane of epithelial cells lining the gastric pit region and at the base of the gastric glands as previously reported in normal human stomach specimens (Sahin et al., 2008; Figure 4A). ARHGAP26 was previously detected on pleiomorphic tubular and punctate membrane structures in HeLa cells (Lundmark et al., 2008). We observed ARHGAP26 in normal stomach on vesicular structures "
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    • "This is not surprising given that GRAF1a will dimerize through its membrane interaction BAR domain while still presenting hydrophobic insertion sequences on each molecule. However, the BAR-PH domain of GRAF1 can also alter local membrane curvature because it induces liposome tubulation in vitro (Lundmark et al., 2008). The presence of a curvature effector on LDs would be consistent with an effect on droplet fusion, as this might well prime an area of the surface, making it fusogenic, as is observed with synaptotagmin during vesicle fusion (Martens et al., 2007). "
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