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Membrane-trafficking sorting hubs: Cooperation between PI4P and small GTPases at the trans-Golgi network

Department of Molecular Biology and Genetics, 107 Biotechnology Bldg., Cornell University, Ithaca, NY 14853-7202, USA.
Trends in cell biology (Impact Factor: 12.31). 07/2011; 21(9):515-25. DOI: 10.1016/j.tcb.2011.05.005
Source: PubMed

ABSTRACT Cell polarity in eukaryotes requires constant sorting, packaging and transport of membrane-bound cargo within the cell. These processes occur in two sorting hubs: the recycling endosome for incoming material and the trans-Golgi network for outgoing material. Phosphatidylinositol 3-phosphate and phosphatidylinositol 4-phosphate are enriched at the endocytic and exocytic sorting hubs, respectively, where they act together with small GTPases to recruit factors to segregate cargo and regulate carrier formation and transport. In this review, we summarize the current understanding of how these lipids and GTPases regulate membrane trafficking directly, emphasizing the recent discoveries of phosphatidylinositol 4-phosphate functions at the trans-Golgi network.

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Available from: Felipe H Santiago-Tirado, Apr 30, 2014
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    • "Phosphatidylinositol 4-kinases (PI4Ks) regulate key aspects of the structure and function of the Golgi apparatus via effector proteins that bind phosphatidylinositol 4-phosphate (PtdIns4P) and coordinate lipid metabolism with vesicle-mediated trafficking pathways (Graham and Burd, 2011; Santiago-Tirado and Bretscher, 2011). PtdIns4P is most abundant on membranes of the trans-Golgi network (TGN; Godi et al., 2004; Cheong et al., 2010), in which PI4Ks reside at steady state (Strahl et al., 2005). "
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    ABSTRACT: In the Golgi apparatus, lipid homeostasis pathways are coordinated with the biogenesis of cargo transport vesicles by phosphatidylinositol 4-kinases (PI4Ks) that produce phosphatidylinositol 4-phosphate (PtdIns4P), a signaling molecule that is recognized by downstream effector proteins. Quantitative analysis of the intra-Golgi distribution of a PtdIns4P reporter protein confirms that PtdIns4P is enriched on the trans-Golgi cisterna, but surprisingly, Vps74 (the orthologue of human GOLPH3), a PI4K effector required to maintain residence of a subset of Golgi proteins, is distributed with the opposite polarity, being most abundant on cis and medial cisternae. Vps74 binds directly to the catalytic domain of Sac1 (K(D) = 3.8 μM), the major PtdIns4P phosphatase in the cell, and PtdIns4P is elevated on medial Golgi cisternae in cells lacking Vps74 or Sac1, suggesting that Vps74 is a sensor of PtdIns4P level on medial Golgi cisternae that directs Sac1-mediated dephosphosphorylation of this pool of PtdIns4P. Consistent with the established role of Sac1 in the regulation of sphingolipid biosynthesis, complex sphingolipid homeostasis is perturbed in vps74Δ cells. Mutant cells lacking complex sphingolipid biosynthetic enzymes fail to properly maintain residence of a medial Golgi enzyme, and cells lacking Vps74 depend critically on complex sphingolipid biosynthesis for growth. The results establish additive roles of Vps74-mediated and sphingolipid-dependent sorting of Golgi residents.
    Molecular biology of the cell 05/2012; 23(13):2527-36. DOI:10.1091/mbc.E12-01-0077 · 5.98 Impact Factor
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    • "The presence of multiple different trafficking pathways for secretion has been described in many cell types (Dikeakos et al. 2007; De Matteis and Luini 2008; Park et al. 2008; Folsch et al. 2009; Perez et al. 2010; Stanley and Lacy 2010; Lacy and Stow 2011; Santiago-Tirado and Bretscher 2011). The best characterized of these pathways have a specific cargo protein that is a unique marker of that route, which is essential for molecular characterization (Lara- Lemus et al. 2006). "
    Crosstalk and Integration of Membrane Trafficking Pathways, 04/2012; , ISBN: 978-953-51-0515-2
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    • "This is perhaps not surprising, given the large size of these GEFs and the importance of ensuring that they are only active in the correct compartment. Several other trans-Golgi proteins bind PtdIns(4)P (Levine and Munro, 2002; Santiago-Tirado and Bretscher, 2011), but overexpression of a pleckstrin homology (PH) domain that binds the Golgi pool of PtdIns(4)P did not displace BIG1, suggesting that the latter may not rely on PI(4)P for recruitment in vivo (Fig. S1 D). Alternatively, Arf1 itself could contribute to Sec71/BIG1 recruitment either via Arf1-GDP binding to the Sec7 domain or by Arf1-GTP recognizing Sec71 as an effector and so enhancing recruitment in a positive feedback loop. "
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    ABSTRACT: The small G protein Arf1 regulates Golgi traffic and is activated by two related types of guanine nucleotide exchange factor (GEF). GBF1 acts at the cis-Golgi, whereas BIG1 and its close paralog BIG2 act at the trans-Golgi. Peripheral membrane proteins such as these GEFs are often recruited to membranes by small G proteins, but the basis for specific recruitment of Arf GEFs, and hence Arfs, to Golgi membranes is not understood. In this paper, we report a liposome-based affinity purification method to identify effectors for small G proteins of the Arf family. We validate this with the Drosophila melanogaster Arf1 orthologue (Arf79F) and the related class II Arf (Arf102F), which showed a similar pattern of effector binding. Applying the method to the Arf-like G protein Arl1, we found that it binds directly to Sec71, the Drosophila ortholog of BIG1 and BIG2, via an N-terminal region. We show that in mammalian cells, Arl1 is necessary for Golgi recruitment of BIG1 and BIG2 but not GBF1. Thus, Arl1 acts to direct a trans-Golgi-specific Arf1 GEF, and hence active Arf1, to the trans side of the Golgi.
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