Trends in Cell Biology
Volume 18, Issue 4, April 2008, Pages 184-192
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Review
Regulation of actin cytoskeleton dynamics by Arf-family GTPases

https://doi.org/10.1016/j.tcb.2008.02.002Get rights and content

Small GTPases of the Arf family are best known for their role in vesicular transport, wherein they nucleate the assembly of coat proteins at sites of carrier vesicle formation. However, accumulating evidence indicates that the Arfs are also important regulators of actin cytoskeleton dynamics and are involved in a variety of actin-based processes, including cell adhesion, migration and neurite outgrowth. The mechanisms of this regulation are remarkably diverse, ranging from the integration of vesicular transport with cytoskeleton assembly to the direct regulation of Rho-family GTPase function. Here, we review recent progress in our understanding of how Arfs and their interacting proteins function to integrate membrane and cytoskeletal dynamics.

Introduction

A dynamic actin cytoskeleton is essential for a wide variety of normal cellular processes, including the maintenance of cell shape and morphology, cytokinesis, adhesion, migration, neurite outgrowth, endocytosis and phagocytosis. Actin remodeling is also important under pathological conditions, such as the invasion and metastasis of cancers. It is well established that actin cytoskeleton dynamics are controlled by small GTPases of the Rho family (for review see [1]). The best characterized of these, RhoA, Rac and Cdc42, interact with distinct sets of downstream effector proteins to direct the assembly of different actin-based structures; RhoA promotes the formation of stress fibers and focal adhesions, Rac induces the formation of lamellipodia and membrane ruffles, and Cdc42 stimulates formation of filopodia. Protrusive structures such as lamellipodia might also require the vectorial addition of new membrane components, such that actin remodeling is often integrated with vesicular transport.

The ADP-ribosylation factor (Arf) proteins are small, ubiquitously expressed GTP-binding proteins best known for their role in membrane trafficking [2]. The six mammalian Arfs are divided into three classes on the basis of sequence relatedness, with class I containing Arf1, Arf2 and Arf3, class II containing Arf4 and Arf5, and class III containing only Arf6. The class I and class II Arfs are mainly concentrated in the Golgi apparatus, although they also function in endosomal compartments. By contrast, Arf6 is localized primarily in the plasma membrane and a subset of endosomes.

A large body of evidence indicates that the Arfs promote carrier vesicle biogenesis by nucleating the assembly of coat protein complexes at sites of vesicle formation. For class I and class II Arfs, these complexes include multimeric coat protein complexes such as COPI (coat protein complex I), AP-1 (adaptor protein complex 1), AP-3 and AP-4, in addition to monomeric adaptors of the GGA (Golgi-associated, gamma adaptin ear-containing and Arf binding) and Mint (Munc18-interacting protein) families [2]. These vesicle coats serve to concentrate cargo proteins in the plane of the membrane and might help to deform the membrane surface, thereby facilitating the formation of carrier vesicles.

In addition to their well characterized role in membrane trafficking, it has become apparent that the Arfs also contribute significantly to the regulation of actin cytoskeletal organization. In this review, we summarize recent progress in our understanding of how Arfs and their interacting proteins can integrate actin cytoskeleton and membrane dynamics.

Section snippets

Arfs, phosphoinositides and the cytoskeleton

One way in which Arfs influence actin cytoskeleton assembly is through their effects on the lipid microenvironment. There is an intimate relationship between phosphoinositides and the actin cytoskeleton. This relationship operates on several levels. First, phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2), binds directly to several important actin regulatory proteins and modulates their function. For example, PtdIns(4,5)P2 stimulates the activity of actin-nucleating factors, such as the

Arf regulation of actin assembly in the Golgi

It has been known for some time that actin and several actin-binding proteins, including spectrin, mAbp1 (mammalian actin-binding protein 1), drebrin, cortactin, N-WASP and the Arp2/3 complex associated with the Golgi (for review, see [10]). It is therefore not surprising that actin-depolymerizing drugs have been shown to disrupt both Golgi morphology and positioning 11, 12. Perhaps more surprising is that most of these proteins rapidly dissociate from the Golgi upon treatment of cells with

Arf regulation of actin dynamics in the cell periphery

Although both class I and class II Arfs are present in the cell periphery, the predominant Arf in this region is Arf6. Expression of dominant–negative mutants or RNAi-mediated depletion indicates that Arf6 plays an important role in a variety of actin-based processes. including phagocytosis, some forms of endocytosis, cell adhesion, migration, invasion of the extracellular matrix. and neurite outgrowth [2]. In a similar fashion to Arf1, Arf6 can activate both phospholipase D and type I PIP5

Crosstalk between Arf6 and Rac

Arf6 can also control cytoskeletal organization through crosstalk with Rho family GTPases. Early studies demonstrated that expression of dominant inhibitory Arf6T27N inhibited membrane ruffling in response to either constitutively active Rac mutants [30] or physiological stimuli [27]. Conversely, activation of endogenous Arf6 by expression of the Arf GEF ARNO (Arf nucleotide-binding site opener) was found to trigger the activation of Rac, extension of lamellipodia and the onset of migration in

Arf-mediated recruitment of Rac GEFs

How does Arf6 activate Rac? In epithelial cells, Arf6 activation leads to recruitment of a bipartite Rac GEF, the Dock180–Elmo complex, to the leading edge (Figure 3a) [39]. Expression of catalytically inactive Dock180, or an Elmo mutant that cannot couple to Dock180, completely inhibits both the activation of Rac and motility downstream of Arf6. Although a direct interaction between Arf6 and Elmo or Dock180 has not yet been demonstrated, an Elmo isoform, ElmoD2, was found to act as a GAP on

The Rac translocation model

Early studies indicated that endogenous Rac1 is present on perinuclear endosomes, and that it can be translocated to the plasma membrane in an Arf6-dependent manner 27, 30. This led to the hypothesis that Arf6 regulates Rac activation by controlling its trafficking from endosomes to the plasma membrane in response to physiological cues (Figure 3b). However, a large cytosolic pool of Rac exists in complex with RhoGDI. In addition to preventing the dissociation of bound GDP, GDIs also enhance the

Arf GAPs as modulators of cytoskeleton dynamics

The human genome encodes 24 proteins with recognizable Arf GAP domains, a subset of which have well-defined interactions with the actin cytoskeleton. Owing to space limitations, it is not possible to review these interactions, or all of the GAP classes, in detail. However, for an excellent and thorough review of the role of Arf GAPs in cytoskeletal dynamics, see [47]. Here, we summarize the features of this group of proteins, members of which impact directly on cytoskeleton function.

Conclusions

In addition to their well-characterized role in vesicular transport, it is now clear that the Arfs have important functions in the regulation of actin cytoskeleton assembly. Such regulation can occur on several levels: (i) the direct, local regulation of phosphoinositide synthesis, which impacts a broad array of actin regulatory proteins; (ii) the activation of Rho family GTPases through the recruitment of GEFs (e.g. Dock180, kalirin or PIX) or trafficking of lipid raft components; (iii) the

Acknowledgements

The authors wish to apologize to our colleagues whose work was not cited, owing to space constraints. We also thank Anne Allison for careful reading of the manuscript, and Lorraine Santy for providing the images used in Figure 2.

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