scholarly journals Localization of Large ADP-Ribosylation Factor-Guanine Nucleotide Exchange Factors to Different Golgi Compartments: Evidence for Distinct Functions in Protein Traffic

2002 ◽  
Vol 13 (1) ◽  
pp. 119-133 ◽  
Author(s):  
Xinhua Zhao ◽  
Troy K.R. Lasell ◽  
Paul Melancİon
Keyword(s):  
Golgi Complex ◽  
Brefeldin A ◽  
Guanine Nucleotide Exchange Factors ◽  
Dependent Manner ◽  
Coat Proteins ◽  
Guanine Nucleotide ◽  
Nucleotide Exchange ◽  
Adp Ribosylation ◽  
Guanine Nucleotide Exchange ◽  
Nucleotide Exchange Factors

Activation of several ADP-ribosylation factors (ARFs) by guanine nucleotide exchange factors (GEFs) regulates recruitment of coat proteins (COPs) on the Golgi complex and is generally assumed to be the target of brefeldin A (BFA). The large ARF-GEFs Golgi-specific BFA resistance factor 1 (GBF1) and BFA-inhibited GEFs (BIGs) localize to this organelle but catalyze exchange preferentially on class II and class I ARFs, respectively. We now demonstrate using quantitative confocal microscopy that these GEFs show a very limited overlap with each other (15 and 23%). In contrast, GBF1 colocalizes with the cis-marker p115 (86%), whereas BIGs overlap extensively with TGN38 (83%). Consistent with these distributions, GBF1, but not BIG1, partially relocalized to peripheral sites after incubation at 15°C. The new GBF1 structures represent peripheral vesicular tubular clusters (VTCs) because 88% of structures analyzed stained for both GBF1 and p115. Furthermore, as expected of VTCs, they rapidly reclustered to the Golgi complex in a microtubule-dependent manner upon warm-up. These observations suggest that GBF1 and BIGs activate distinct subclasses of ARFs in specific locations to regulate different types of reactions. In agreement with this possibility, COPI overlapped to a greater extent with GBF1 (64%) than BIG1 (31%), whereas clathrin showed limited overlap with BIG1, and virtually none with GBF1.

scholarly journals Distinct Functions for Arf Guanine Nucleotide Exchange Factors at the Golgi Complex: GBF1 and BIGs Are Required for Assembly and Maintenance of the Golgi Stack and trans-Golgi Network, Respectively

2008 ◽  
Vol 19 (2) ◽  
pp. 523-535 ◽  
Author(s):  
Florin Manolea ◽  
Alejandro Claude ◽  
Justin Chun ◽  
Javier Rosas ◽  
Paul Melançon
Keyword(s):  
Golgi Complex ◽  
Brefeldin A ◽  
Adaptor Proteins ◽  
Guanine Nucleotide Exchange Factors ◽  
Coat Proteins ◽  
Guanine Nucleotide ◽  
Nucleotide Exchange ◽  
Golgi Stack ◽  
Guanine Nucleotide Exchange ◽  
Nucleotide Exchange Factors

We examined the relative function of the two classes of guanine nucleotide exchange factors (GEFs) for ADP-ribosylation factors that regulate recruitment of coat proteins on the Golgi complex. Complementary overexpression and RNA-based knockdown approaches established that GBF1 regulates COPI recruitment on cis-Golgi compartments, whereas BIGs appear specialized for adaptor proteins on the trans-Golgi. Knockdown of GBF1 and/or COPI did not prevent export of VSVGtsO45 from the endoplasmic reticulum (ER), but caused its accumulation into peripheral vesiculotubular clusters. In contrast, knockdown of BIG1 and BIG2 caused loss of clathrin adaptor proteins and redistribution of several TGN markers, but had no impact on COPI and several Golgi markers. Surprisingly, brefeldin A–inhibited guanine nucleotide exchange factors (BIGs) knockdown prevented neither traffic of VSVGtsO45 to the plasma membrane nor assembly of a polarized Golgi stack. Our observations indicate that COPII is the only coat required for sorting and export from the ER exit sites, whereas GBF1 but not BIGs, is required for COPI recruitment, Golgi subcompartmentalization, and cargo progression to the cell surface.

Arf, Sec7 and Brefeldin A: a model towards the therapeutic inhibition of guanine nucleotide-exchange factors

2005 ◽  
Vol 33 (6) ◽  
pp. 1265-1268 ◽  
Author(s):  
M. Zeghouf ◽  
B. Guibert ◽  
J.-C. Zeeh ◽  
J. Cherfils
Keyword(s):  
Exchange Reaction ◽  
G Proteins ◽  
Brefeldin A ◽  
Membrane Traffic ◽  
Guanine Nucleotide Exchange Factors ◽  
Guanine Nucleotide ◽  
Nucleotide Exchange ◽  
Guanine Nucleotide Exchange ◽  
Nucleotide Exchange Factors ◽  
Small G Proteins

GEFs (guanine nucleotide-exchange factors), which stimulate GDP dissociation from small G-proteins, are pivotal regulators of signalling pathways activated by small G-proteins. In the case of Arf proteins, which are major regulators of membrane traffic in the cell and have recently been found to be involved in an increasing number of human diseases, GDP/GTP exchange is stimulated by GEFs that carry a catalytic Sec7 domain. Recent structural results captured snapshots of the exchange reaction, revealing that Sec7 domains secure Arf-GDP to membranes before nucleotide exchange takes place, taking advantage of a built-in structural device in Arf proteins that couples their affinity for membranes to the nature of the bound nucleotide. One of the Arf–Sec7 intermediates was trapped by BFA (Brefeldin A), an uncompetitive inhibitor of Arf activation that has been instrumental in deciphering the molecular principles of membrane traffic at the Golgi. BFA targets a low-affinity Arf–Sec7 intermediate of the exchange reaction. It binds at the Arf-GDP/Sec7 interface, thus freezing the complex in an abortive conformation that cannot proceed to nucleotide dissociation. In the cell, this results in the specific inhibition of Arf1 by a subset of its GEFs, and the efficient and reversible block of membrane traffic at the Golgi. The mechanism of BFA leads to the concept of ‘interfacial inhibition’, in which a protein–protein interaction of therapeutic interest is stabilized, rather than impaired, by a drug. Up-regulated activity of small G-proteins is involved in various human diseases, making their GEFs attractive candidates to interrupt specifically the corresponding signalling pathway. Interfacial inhibitors are proposed as an alternative to competitive inhibitors that may be explored for their inhibition.

scholarly journals Redundant Roles of BIG2 and BIG1, Guanine-Nucleotide Exchange Factors for ADP-Ribosylation Factors in Membrane Traffic between the trans-Golgi Network and Endosomes

2008 ◽  
Vol 19 (6) ◽  
pp. 2650-2660 ◽  
Author(s):  
Ray Ishizaki ◽  
Hye-Won Shin ◽  
Hiroko Mitsuhashi ◽  
Kazuhisa Nakayama
Keyword(s):  
Membrane Traffic ◽  
Guanine Nucleotide Exchange Factors ◽  
Guanine Nucleotide ◽  
Nucleotide Exchange ◽  
Recycling Endosomes ◽  
Adp Ribosylation ◽  
Trans Golgi Network ◽  
Guanine Nucleotide Exchange ◽  
Nucleotide Exchange Factors ◽  
Golgi Network

BIG2 and BIG1 are closely related guanine-nucleotide exchange factors (GEFs) for ADP-ribosylation factors (ARFs) and are involved in the regulation of membrane traffic through activating ARFs and recruiting coat protein complexes, such as the COPI complex and the AP-1 clathrin adaptor complex. Although both ARF-GEFs are associated mainly with the trans-Golgi network (TGN) and BIG2 is also associated with recycling endosomes, it is unclear whether BIG2 and BIG1 share some roles in membrane traffic. We here show that knockdown of both BIG2 and BIG1 by RNAi causes mislocalization of a subset of proteins associated with the TGN and recycling endosomes and blocks retrograde transport of furin from late endosomes to the TGN. Similar mislocalization and protein transport block, including furin, were observed in cells depleted of AP-1. Taken together with previous reports, these observations indicate that BIG2 and BIG1 play redundant roles in trafficking between the TGN and endosomes that involves the AP-1 complex.

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