Arf and its many interactors

doi:10.1016/S0955-0674(03)00071-1

Current Opinion in Cell Biology

Volume 15, Issue 4, August 2003, Pages 396-404

https://doi.org/10.1016/S0955-0674(03)00071-1 Get rights and content

Abstract

Arf GTP-binding proteins regulate membrane traffic and actin remodeling. Similar to other GTP-binding proteins, a complex of Arf–GTP with an effector protein mediates Arf function. Arf interacts with at least three qualitatively different types of effectors. First, it interacts with structural proteins, the vesicle coat proteins. The second type of effector is lipid-metabolizing enzymes, and the third comprises those proteins that bind to Arf–GTP but whose biochemical or biological functions are not yet clearly defined. Arf interacts with two other families of proteins, the exchange factors and the GTPase-activating proteins. Recent work examining the functional relationships among the diverse Arf interactors has led to reconsideration of the prevailing paradigms for Arf action.

Introduction

ADP-ribosylation factor (Arf)-family proteins are Ras-like GTP-binding proteins of ∼20 kDa 1., 2., 3., 4., 5., 6.. They are found in all eukaryotic organisms examined to date and are highly conserved. Mammals have six Arfs that are categorized into three classes based on primary structure: Arfs 1, -2 and -3 comprise class I, Arfs -4 and –5, class II, and Arf6, class III. Arf1 and Arf6 are the most extensively characterized of the Arf proteins, and both have been implicated at multiple sites as both regulators of membrane traffic and actin polymerization.

The effects of Arfs depend on their function as GTP-dependent switches. Analogous to other Ras-like proteins, the conformation of two regions of Arf (Figure 1), switch 1 (residues 40–51 of Arf1) and 2 (residues 68–81 of Arf1), differ between the GDP- and GTP-bound forms 7.••, 8., 9., 10., 11., 12., 13., 14., 15.. Arf has two additional domains that are sensitive to bound nucleotide. One is the myristoylated amino-terminal amphipathic helix (residues 2–17 in Arf1) that associates with a hydrophobic cleft in Arf–GDP but is free to associate with lipids and presumably other proteins in Arf–GTP. The second is the interswitch domain (residues 52–67 in Arf1) that is covered by the myristoylated amino terminus in Arf–GDP but is surface-exposed in Arf–GTP. These four surfaces on Arf presumably form the interface for nucleotide-dependent association with other proteins. The carboxyl terminus [16] and α-helix 3 (residues 100–112 in Arf1) [8] have also been implicated in protein–protein interactions.

Arf function in cells depends on several classes of proteins. Arf–GDP interaction with guanine nucleotide exchange factors (GEFs) promotes the formation of Arf–GTP 2., 17., 18., 19.. GTPase-activating proteins (GAPs) are negative regulators that recognize Arf–GTP and induce hydrolysis of GTP 2., 4., 20.. Arf–GTP bound to effector proteins mediates the physiological functions of Arf 2., 4.. Consistent with the multiple sites of action and range of effects, Arf–GTP interacts with a diverse group of proteins, including vesicle coat proteins and lipid-metabolizing enzymes.

A model for Arf regulation of membrane trafficking through interaction with coat proteins is well established. The role of other Arf-binding proteins in Arf function and the relationship among the Arf-binding proteins remain elusive, however. Here, we review recent literature about the biochemical and biological function of Arf-interacting proteins. The results discussed provide insights into the molecular mechanisms underlying Arf function as a regulator of membrane trafficking and the actin cytoskeleton.

Section snippets

Arf–GDP-interacting proteins

Two classes of proteins interact with Arf–GDP. One class, nucleotide exchange factors, has been examined extensively and is the subject of numerous reviews 2., 17., 18., 19.. The mammalian Arf GEFs comprise a family of 14 proteins in five subfamilies (Table 1). All contain a Sec7 domain that catalyzes the exchange of nucleotide. Crystallographic studies revealed that switch 1 and 2 of Arf form an interface with the Sec7 domain 9., 21., 22., 23., 24., 25.. Nucleotide dissociation is favored by

Effectors

Arf–GTP bound to effector proteins mediates Arf activity. To understand Arf function, screens for Arf–GTP-binding proteins have been performed. Consistent with the multiple sites of action and diverse effects of Arfs, a diverse group of proteins have been identified. The proteins fit three categories (Table 2), as discussed below.

GTPase-activating proteins

Sixteen mammalian Arf GAPs have been identified (Table 1) as well as several proteins containing the Arf GAP motif but without demonstrated Arf GAP activity [4]. The Arf GAPs have been categorized into three groups: Arf GAP1 type 69., 70., Git type 71., 72. and AZAP type 4., 73.. The AZAPs have been further divided into four subgroups, the ASAPs 73., 74., AGAPs [75], ARAPs 76., 77. and ACAPs [78]. The structurally complex Arf GAPs are targets of several signaling cascades and have functions in

Functional relationships among Arf-interacting proteins

Biological responses require controlled GTP-binding to Arf, interaction of Arf–GTP with appropriate effector(s) and appropriately timed inactivation of Arf. The coordination of these biochemical activities appears to involve direct interaction among Arf-interacting molecules. One example is the complex relationships among three proteins, Arf GAP1, coatomer and the cargo receptor p24a. Arf GAP1 binds directly to coatomer 8., 28.••, 82.. The two proteins also bind Arf1 simultaneously — Arf GAP1

Conclusions

The identification of diverse group of proteins that comprise the Arf pathway has led to significant progress in understanding the cellular roles of Arfs. Previously viewed as a mediator of recruitment to membranes, Arf might function by directly regulating specific protein interactions, as proposed for Arfaptin–Rac and GGA–MPR. Interactions between Arf effectors and regulators are also being incorporated into models that explain how Arfs control membrane trafficking. Plausible mechanisms for

Update

The relationship between GGA and AP1 might involve more than a simple ‘hand-off’ of cargo. Instead, GGA and AP1 functioning together could produce a unique membrane-traffic intermediate. A recent paper [95^•] identified a distinct cargo ‘container’ involved in transport between the TGN and the endosomes. The structures contained GGA, AP1 and clathrin, were larger than typical clathrin/AP vesicles and were pleiomorphic. GGA remained associated with the structures until fusion with endosomes. The

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

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of special interest
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of outstanding interest

Acknowledgements

We thank Douglas R Lowy and the Center for Cancer Research at the National Cancer Institute for continued support. We thank our colleagues, especially Douglas R Lowy, Juan S Bonifacino, Julie G Donaldson and Catherine L Jackson, for discussions about the biology of Arf. We apologize to any colleagues whose work, because of either space constraints or our own limitations, may have been excluded or overlooked. We hope the deficiencies will not preclude the intended purpose of this paper; that is,

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Published by Elsevier Ltd.

Arf and its many interactors

Abstract

Introduction

Section snippets

Arf–GDP-interacting proteins

Effectors

GTPase-activating proteins

Functional relationships among Arf-interacting proteins

Conclusions

Update

References and recommended reading

Acknowledgements

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