Sequential interactions with Sec23 control the direction of vesicle traffic

Department of Cellular and Molecular Medicine, Howard Hughes Medical Institute, University of California at San Diego, La Jolla, California 92093-0668, USA.
Nature (Impact Factor: 41.46). 05/2011; 473(7346):181-6. DOI: 10.1038/nature09969
Source: PubMed


How the directionality of vesicle traffic is achieved remains an important unanswered question in cell biology. The Sec23p/Sec24p coat complex sorts the fusion machinery (SNAREs) into vesicles as they bud from the endoplasmic reticulum (ER). Vesicle tethering to the Golgi begins when the tethering factor TRAPPI binds to Sec23p. Where the coat is released and how this event relates to membrane fusion is unknown. Here we use a yeast transport assay to demonstrate that an ER-derived vesicle retains its coat until it reaches the Golgi. A Golgi-associated kinase, Hrr25p (CK1δ orthologue), then phosphorylates the Sec23p/Sec24p complex. Coat phosphorylation and dephosphorylation are needed for vesicle fusion and budding, respectively. Additionally, we show that Sec23p interacts in a sequential manner with different binding partners, including TRAPPI and Hrr25p, to ensure the directionality of ER-Golgi traffic and prevent the back-fusion of a COPII vesicle with the ER. These events are conserved in mammalian cells.

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Available from: Majid Ghassemian
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    • "Sec16A (hereafter called Sec16) plays an important role in ERES homeostasis as it interacts with several COPII components and regulates their function (Bhattacharyya and Glick, 2007; Connerly et al., 2005; Farhan et al., 2008; Ivan et al., 2008; Supek et al., 2002; Watson et al., 2006; Espenshade et al., 1995; Gimeno et al., 1996; Whittle and Schwartz, 2010; Kung et al., 2012). Various post-translational modifications, such as phosphorylation (Farhan et al., 2010; Koreishi et al., 2013; Lord et al., 2011; Sharpe et al., 2011), ubiquitylation (Jin et al., 2012) or glycosylation (Dudognon et al., 2004), were reported to modulate ER export, but the molecular details and functional consequences of this regulation remain to be established. Several questions remain unanswered: how does phosphorylation of Sec16 (Farhan et al., 2010) lead to changes in ERES? "
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    ABSTRACT: We currently lack a broader mechanistic understanding of the integration of the early secretory pathway with other homeostatic processes such as cell growth. Here, we explore the possibility that Sec16A, a major constituent of endoplasmic reticulum exit sites (ERES), acts as an integrator of growth factor signalling. Surprisingly, we find that Sec16A is a short-lived protein that is regulated by growth factors in a manner dependent on Egr family transcription factors. We hypothesize that Sec16A acts as a central node in a coherent feed-forward loop that detects persistent GF stimuli to increase ERES number. Consistent with this notion, Sec16A is also regulated by short-term growth factor treatment that leads to increased turnover of Sec16A at ERES. Finally, we demonstrate that Sec16A depletion reduces, while its overexpression increases proliferation. Together with our finding that growth factors regulate Sec16A levels and its dynamics on ERES, we propose this protein as an integrator linking growth factor signalling and secretion. This provides a mechanistic basis for the previously proposed link between secretion and proliferation.
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    • "It is possible that during budding and/or just before completion, at least partial portion of the Sar1/Arf1 proteins were released from the membrane surface of coated vesicles by the action of GAP. This fits with the observation that the TRAPPI complex and Ypt1, which serve in the tethering event, can bind to COPII vesicles through the interaction with Sec23 after Sar1 is depleted (Cai et al., 2007; Lord et al., 2011). Subsequently, at the Golgi surface, the Hrr25 protein kinase, in association with the Golgi, phosphorylates Sec23/24 to release the coat and eventually promote vesicle fusion. "
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    • "Consistent with their functional importance, we recently confirmed that the C-terminal tail of TgSORTLR binds specifically to the cytosolic sorting complexes involved in anterograde transport or retrograde transport [45]. For anterograde transport, the TgSORTLR cytoplasmic tail not only binds clathrin and to three components of the AP1 and AP2 adaptor complexes, but also to homologues of Vps9 and of the COPII or coat complex transport proteins Sec23/Sec24 that ensure the directionality of anterograde membrane flow from the ER to the Golgi apparatus [47]. For retrograde transport, TgSORTLR binding to Vps26-Vps29-Vps35 indicated its association with the retromer complex, which mediates transport from endosomes to the trans-Golgi network. "
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