Arf GAPs and membrane traffic

Abstract

The selective transfer of material between membrane-delimited organelles is mediated by protein-coated vesicles. In many instances, formation of membrane trafficking intermediates is regulated by the GTP-binding protein Arf. Binding and hydrolysis of GTP by Arf was originally linked to the assembly and disassembly of vesicle coats. Arf GTPase-activating proteins (GAPs), a family of proteins that induce hydrolysis of GTP bound to Arf, were therefore proposed to regulate the disassembly and dissociation of vesicle coats. Following the molecular identification of Arf GAPs, the roles for GAPs and GTP hydrolysis have been directly examined. GAPs have been found to bind cargo and known coat proteins as well as directly contribute to vesicle formation, which is consistent with the idea that GAPs function as subunits of coat proteins rather than simply Arf inactivators. In addition, GTP hydrolysis induced by GAPs occurs largely before vesicle formation and is required for sorting. These results are the primary basis for modifications to the classical model for the function of Arf in transport vesicle formation, including a recent proposal that Arf has a proofreading, rather than a structural, role.

ArfGAP1 promotes COPI vesicle formation by facilitating coatomer polymerization

“…Two opposing models are currently debated. 4,17,18,34,37,38,48 In one model, ArfGAP1 acts as a negative regulator of the GTP-binding protein Arf, preventing formation of vesicles and/or triggering coat dissociation from vesicles. In a second model, ArfGAP1 has a positive role for the formation of vesicles.…”

“…Lipofectamine™ 2000 was used for NIH3T3 cells. After 18-22 h, cells were washed with PBS and fixed with 4% paraformaldehyde (PFA) for 15 min and quenched with 50 mM NH 4 Cl. Cells were incubated with blocking solution A (0.04% Saponin, 0.2% BSA, in PBS) for 30 min.…”

“…[1][2][3] Five ArfGAPs associate with the Golgi apparatus. 2,[4][5][6][7] Three, ArfGAP1, 2 and 3, regulate traffic between the Golgi apparatus and the endoplasmic reticulum (ER). The function of ArfGAP1, 2 and 3 is, at least in part, related to interaction with the substrate protein Arf1•GTP, vesicle coat protein and the cargo proteins that are being transported.…”

“…[8][9][10][11][12][13][14][15] The molecular basis for retrograde traffic between the Golgi cisternae or from the Golgi apparatus to the ER has been extensively studied. 2,[4][5][6][7]16 In these pathways, cargo is transferred from trans-side cisternae to the more cis-side, or from the Golgi apparatus to the endoplasmic reticulum (ER). Cargo proteins include p24 family proteins (p23, p24, p25 and p26) and the KDEL receptor.…”

“…The conversion of Arf1•GDP to Arf1•GTP results in the recruitment of coatomer to membranes containing cargo. 4,17,18 Coatomer binds to cargo and then polymerizes into COPI, which deforms the membrane into a bud. The coated bud undergoes fission to…”