Cell motility

Actin branches out

The ability of cells to move is essential for the development and survival of multicellular organisms, yet it can also be detrimental — when cancer cells migrate to distant parts of the body and colonize new tumours, for example. Cell movement is thought to be driven largely by the polymerization of actin monomers into filaments near the plasma membrane, and by the contractile forces of myosin motors found deeper in the cell. Although the mechanism of muscle contraction is fairly well understood, we still don't know much about the roles of actin and myosin in cell migration. But reports by Mullins et al.1 in Proceedings of the National Academy of Sciences and Welch et al.2 in Science reveal that a seven-protein complex is central in organizing actin structures. This complex contains the actin-related proteins Arp2 and Arp3, and has been thrust into the spotlight because it may initiate and organize protrusion of an actin-based dynamic meshwork into thin sheets (lamellipodia) or spikes (filopodia).

First identified in Acanthamoeba3, the Arp2/3 complex is conserved from yeast to mammals. In fibroblasts and amoebas, this complex is localized at the leading edge of cells — the ideal position to initiate the polymerization of actin that is required for cell motility. However, the Arp2/3 complex is not found in the more stable actin-myosin bundles that are normally associated with adhesion and contraction4,5,6. In budding yeast, the Arp2/3 complex is required for motility of the cortical actin patches (dynamic, actin-rich structures that may be analogous to mammalian cortical actin) in which it is found7,8. It is also the only known host-encoded factor that can induce actin polymerization on the surface of the motile intracellular pathogen Listeria monocytogenes9.

Mullins et al.1 and Welch et al.2 now explain why the Arp2/3 complex is associated with dynamic actin structures. They find that it seeds, or ‘nucleates’, actin filaments to polymerize from their barbed (fast-growing) ends (Fig. 1). This has led Mullins et al. to propose an attractive model for the mechanism of actin polymerization at the leading edge of the cell (Fig. 2). In this model, an actin dimer elongates at its barbed end while being capped at its pointed (slow-growing) end by the Arp2/3 complex (Fig. 2). The authors call this mechanism ‘dendritic nucleation’ because the in vitro data1 indicate that new filaments could be nucleated off the sides of old ones, as well as being made from scratch (inset in Fig. 3, overleaf). Interestingly, Svitkina et al.10 have observed similar actin branches in lamellipodia of fibroblasts (Fig. 3) and motile keratocytes. These authors proposed that branches might be nucleated by the Arp2/3 complex and drive protrusion of the lamellipodia.

Figure 1: Structure of an actin filament.
figure1

The filament has polarity, conferred by its structure and by the bound adenine nucleotide. The filament shown is decorated with the S1 sub-fragment of myosin II, which binds in a polarized fashion (yellow arrowheads). The barbed side of the arrow represents the fast-growing end of the filament, whereas the point of the arrowhead represents the depolymerizing end. Mullins et al.1 have found that the Arp2/3 complex caps the pointed end with high affinity. Bound ATP is hydrolysed randomly, so the older regions of the filament (nearer the pointed end) have mostly ADP bound. (Reprinted in modified form with permission from Svitkina and Borisy15.)

Figure 2: Dendritic-nucleation model for protrusion of lamellipodia.
figure2

a, The Arp2/3 complex is targeted to sites where the cell has received a signal to form a lamellipodium. b, The newly activated Arp2/3 complex nucleates actin filaments off the sides of pre-existing filaments to form a branched network of actin, which drives protrusion of the leading edge. c, Actin filaments, capped at their pointed ends by the Arp2/3 complex, can elongate transiently and rapidly at their free barbed ends. d, The rapid growth is terminated by capping of the barbed ends, imposing an additional regulation on the elongating filaments. e, As ATP in the actin filaments is hydrolysed to ADP, the filaments become more susceptible to being severed by cofilin. f, Severing promotes disassembly, mainly in ‘older’ regions of the filaments, closer to the cell body. g, The actin-monomer-binding protein profilin could then bind to the newly released ADP-actin monomers, promoting nucleotide exchange and recharging them for polymerization into new filaments. (Reprinted in modified form from Mullins et al.1.)

Figure 3: Electron micrograph showing actin filaments and bundles in the lamellipodium of a fibroblast.
figure3

D. MULLINS & T. SVITKINA

The inset shows Arp2/3 complex mixed with gelsolin-capped actin filaments in vitro. The branch points both in vitro (inset) and in vivo(white box) have a similar fixed angle and seem to contain one Arp2/3 complex at the junction.

Pointed-end capping and branching by the Arp2/3 complex add a new dimension to the way we think about actin dynamics at the leading edge of motile cells. Previously, the barbed ends of actin filaments near the plasma membrane were thought to be blocked by the binding of barbed-end capping proteins until the caps dissociated in response to extracellular signals11. Once free, these barbed ends then grew rapidly until they were re-capped. In this model, the pointed ends of the actin filament were always assumed to be free. But the new model of dendritic nucleation suggests that actin polymerization occurs not only by uncapping, but also by Arp2/3-driven nucleation of filaments with free barbed ends and capped pointed ends (Fig. 2).

The interplay between nucleation and uncapping, as well as the branching and bundling activities of the Arp2/3 complex, may help to explain the relationship between subtly different structures at the leading edge of cells. Motile cells such as amoebas, leukocytes and growing neurons assemble both filopodia and lamellipodia at their leading edges. Filopodia are thought to sense the environment and direct polarized movement, whereas the actin-rich lamellipodia protrude from the cell in the direction of movement. If the Arp2/3 complex regulates the interplay between orthogonal (lamellipodia) and parallel (filopodia) networks, as well as controlling rates of actin assembly, it could be a control centre for actin dynamics and organization at the leading edge. This may be similar to the way in which a focal adhesion is thought to control both the sensing and mechanics of cell adhesion.

The dendritic-nucleation model also means that we must revise our ideas about turnover of actin filaments in motile cells. The density of actin filaments is highest at the leading edge of a lamellipodium, gradually decreasing towards the middle of the cell10,12. This was previously attributed to disassembly of the free pointed ends nearer the centre at a fairly constant rate, as well as the action of cofilin — a protein that depolymerizes and severs actin filaments13. But if the Arp2/3 complex caps all of the pointed ends, these ends can only be disassembled if the actin filament is first severed by proteins such as cofilin. Because of ATP hydrolysis, older regions of an actin filament tend to contain bound ADP, so they are more susceptible to depolymerization or being severed by cofilin. ATP hydrolysis on actin could also mediate dissociation of the Arp2/3 complex from pointed ends, triggering rapid depolymerization. This dynamic disassembly could be important for cellular retractions or rapid changes in direction.

Few aspects of actin dynamics seem to be untouched by the activities of the Arp2/3 complex. But several questions remain. How do actin filaments turn over in the cell, given the high concentrations of both barbed- and pointed-end capping proteins? How does cofilin cooperate with the Arp2/3 complex to promote disassembly? And how is the Arp2/3 complex regulated, both temporally and spatially? Welch et al.2 may have already set us crawling in the right direction by their observation that the amino-terminal half of the ActA protein from Listeria causes a 50-fold increase in the nucleation activity of the Arp2/3 complex. ActA is the only bacterially encoded protein that is needed for actin polymerization on the surface of Listeria14, and the authors suggest that some cellular proteins may be functionally homologous to the amino terminus of ActA. These proteins could regulate the activity of the Arp2/3 complex in an analogous fashion. So, the race is now on to find what regulates the Arp2/3 complex in the cell and to understand how this complex impinges on cell motility through its various activities.

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Machesky, L., Way, M. Actin branches out. Nature 394, 125–126 (1998). https://doi.org/10.1038/28039

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