Regulating the large Sec7 ARF guanine nucleotide exchange factors: the when, where and how of activation

Abstract

Eukaryotic cells require selective sorting and transport of cargo between intracellular compartments. This is accomplished at least in part by vesicles that bud from a donor compartment, sequestering a subset of resident protein “cargos” destined for transport to an acceptor compartment. A key step in vesicle formation and targeting is the recruitment of specific proteins that form a coat on the outside of the vesicle in a process requiring the activation of regulatory GTPases of the ARF family. Like all such GTPases, ARFs cycle between inactive, GDP-bound, and membrane-associated active, GTP-bound, conformations. And like most regulatory GTPases the activating step is slow and thought to be rate limiting in cells, requiring the use of ARF guanine nucleotide exchange factor (GEFs). ARF GEFs are characterized by the presence of a conserved, catalytic Sec7 domain, though they also contain motifs or additional domains that confer specificity to localization and regulation of activity. These domains have been used to define and classify five different sub-families of ARF GEFs. One of these, the BIG/GBF1 family, includes three proteins that are each key regulators of the secretory pathway. GEF activity initiates the coating of nascent vesicles via the localized generation of activated ARFs and thus these GEFs are the upstream regulators that define the site and timing of vesicle production. Paradoxically, while we have detailed molecular knowledge of how GEFs activate ARFs, we know very little about how GEFs are recruited and/or activated at the right time and place to initiate transport. This review summarizes the current knowledge of GEF regulation and explores the still uncertain mechanisms that position GEFs at “budding ready” membrane sites to generate highly localized activated ARFs.

Cell-Specific Establishment of Poliovirus Resistance to an Inhibitor Targeting a Cellular Protein

“…GBF1 is a guanine nucleotide exchange factor for several small cellular GTPases of the Arf family, and in uninfected cells the protein organizes the formation of COPI transport vesicles transferring cargo between cis-Golgi membranes and the endoplasmic reticulum. It also coordinates recruitment of other proteins to the Golgi membranes, and its activity is important for maintaining the overall organization of the Golgi structure (39,40). It was proposed that Arf-activating function of GBF1 was important for enterovirus replication and that enrichment of activated Arf on the replication membranes may be required to attract other cellular factors supporting replication such as the phosphatidylinositol kinase PI4KIII␤ (41).…”

Allosteric regulation of Arf GTPases and their GEFs at the membrane interface

“…8 Conserved domains outside the Sec7 domain in these ArfGEFs were first identified based on sequence analysis. 34 They include the DCB and HUS domains upstream of the Sec7 domain and the HDS1 to HDS3 domains downstream of the Sec7 domain, none of them has homology to any conventional membrane-binding domain (Fig.…”

“…Arf GTPases are activated by GEFs characterized by a conserved Sec7 domain which is responsible for stimulating GDP/GTP exchange. 4,5 Two major families of ArfGEFs in eukaryotes and one bacterial family can be defined based on their related domain organizations [6][7][8] ( Fig. 1).…”

The Golgi apparatus: insights from filamentous fungi

“…Yeast early Golgi Gea1p has been shown to interact with a subunit of CopI (Deng et al 2009), an Arf1 effector that has roles in intra-Golgi and Golgi-to-ER traffic. Apart from their largely unexplored roles, the specific membrane recruitment mechanism of Golgi Arf1 GEFs (they are peripheral membrane proteins) to specialized Golgi subcompartments is not understood (Wright et al 2014).…”