ReviewStructure and function of the Arp2/3 complex
Introduction
Motile eukaryotic cells depend on the assembly of actin filaments at their leading edge to push the plasma membrane and the cell forward. This process is regulated by external signals such as chemotactic factors, which guide cells toward their destinations by triggering the assembly of a branched network of actin filaments just beneath the plasma membrane. As these filaments grow, they push the membrane forward. Many guidance signals converge on proteins that activate a macromolecular assembly of proteins called the Arp2/3 complex. Active Arp2/3 complex directs the formation of a new filament as a 70° branch on the side of an existing filament, giving the new filament a base from which to push forward. The Arp2/3 complex is also required for the assembly of actin filament patches associated with the plasma membrane of fungal cells.
This article reviews recent work on the structure and function of the Arp2/3 complex. A 2.0 Å crystal structure of the bovine Arp2/3 complex and cryo-electron microscopic structures of the activated Arp2/3 complex and branch junctions have illuminated recent biochemical and genetic studies of the structure and mechanism of action of the complex.
Section snippets
Role of the Arp2/3 complex in cellular actin filament assembly
The Arp2/3 complex was discovered independently several times over a period of a few years following its initial isolation from Acanthamoeba by affinity chromatography on the actin-binding protein profilin [1]. The original study identified seven tightly associated subunits, including actin-related proteins 2 and 3 (called Arp2 and Arp3), a 40 kDa protein with sequence similarity to a WD40 β-propeller protein and four novel subunits. These subunits are commonly named according to their size,
Structure and mechanism of action of the Arp2/3 complex
Determination of the native molecular weight established that the Arp2/3 complex consists of one copy of each of the seven subunits 3., 49.. Chemical cross-linking 49., 50. yeast two-hybrid analysis 14., 51. and genetic experiments 52., 53. led to a general model of the subunit arrangement of the Arp2/3 complex [54]. Although remarkably accurate in many respects, these nearest-neighbor models provided only limited insight into the mechanism of actin filament nucleation and branching.
Recent
Conclusions
Flourishing structural, biochemical, cellular and mathematical research on the Arp2/3 complex has advanced our understanding of the molecular basis of cellular motility, but raises several challenging questions. The mechanism of branching nucleation will not be understood until additional atomic structures are available, particularly of activated Arp2/3 complex with bound nucleotides and nucleation-promoting factors. A combination of structural approaches will probably be required to learn how
Update
Chemical cross-linking established the interaction of the nucleation-promoting factor cortactin with the Arp3 subunit of the Arp2/3 complex and confirmed the interaction of N-WASP VCA with Arp2, Arp3 and ARPC1 [70]. N-WASP VCA and cortactin competed for Arp3, but VCA could still interact with Arp2 and p40, yielding an active ternary complex.
Acknowledgements
Our work on the Arp2/3 complex was supported by National Institutes of Health research grant GM-26338.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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