Actin-binding proteins

Abstract

Actin is an essential component of the cytoskeleton and plays a crucial role in eukaryotic cells. The actin cytoskeleton functions in the generation and maintenance of cell morphology and polarity, in endocytosis and intracellular trafficking, in contractility, motility and cell division. In cells, the assembly and disassembly of actin filaments, and also their organisation into functional higher-order networks, is regulated by a plethora of actin-binding proteins (ABPs) (dos Remedios et al., 2003; Maciver, 2004). The activities of these proteins are in turn under the control of specific signalling pathways.Here, we aim to highlight the main classes of ABP found in eukaryotic cells and indicate their likely mechanism of action as far as possible on the basis of both in vitro and in vivo studies.Actin exists in two principal forms, globular, monomeric (G) actin, and filamentous polymeric (F) actin. Because of the arrowhead pattern observed when myosin decorates actin filaments, the fast-growing end of the polarised polymer is denoted the barbed end and the slow-growing end is donoted pointed end. Actin filaments extend when ATP-actin monomers are preferentially incorporated at the barbed end. As the filament matures, ATP bound in the central cleft of actin is hydrolysed, phosphate is released and the resulting ADP-actin filament is disassembled by loss of monomers from the pointed end. The released ADP-actin monomers then undergo nucleotide exchange to generate ATP-actin monomers that can be used for new rounds of polymerisation. This ATP-hydrolysis-driven, directional filament-growth is called actin treadmilling.The first stage in de novo filament formation is nucleation. Polymerisation is energetically unfavourable until there is a nucleus of three associating monomers and in vitro this stage of filament formation is denoted the lag phase. In vivo, ABPs are crucial to ensure the rapid nucleation of filaments and these essentially function to remove the lag period. New filaments can form de novo, from the side of existing filaments, or by severing an existing filament. Different proteins function to promote each of these modes of nucleation.The Arp2/3 complex can nucleate filaments from the side of existing filaments. This is important to allow the dendritic branching that is found at the leading edge of motile cells (Pollard and Borisy, 2003). Arp2 and Arp3 molecules themselves have a similar tertiary structure to actin itself, and when the Arp2/3 complex binds G-actin the effect is to generate a stable trimer and nucleus for the growth of a filament. Arp2/3 then acts as a pointed-end-capping protein that encourages rapid growth of the filament from its barbed end. Whereas the Arp2/3 complex can nucleate new filaments in vitro, it is likely that its activity in vivo is enhanced by functional interactions with other proteins, most importantly the WASP and SCAR/WAVE proteins. The G-actin-binding WASP homology2 (WH2) domain of WASP is important for the actin-nucleating activity of Arp2/3 and its role is probably to feed actin monomers to the filament barbed end. Verprolin/WIP binds both monomers and F-actin and has been shown to enhance Arp2/3-mediated actin polymerisation when bound to the Arp2/3 activator cortactin (reviewed in Paavilainen et al., 2004).Actin polymerisation is also nucleated by the formin proteins. These proteins have only recently come under close scrutiny and there is still much to be understood about their mechanism of action in cells. However, studies indicate that the formin homology2 (FH2) domain dimer flexes to accommodate the processive addition of actin monomers to the barbed end of a filament (Xu et al., 2004). In yeast it is thought that the long actin cables that are required for cell polarity are generated by the nucleating action of the formins whereas the short branched networks in actin patches are generated by Arp2/3 nucleation.Once nucleated, actin filaments are able to grow rapidly by addition of monomers at their barbed ends. Filaments are controlled by several mechanisms. Filament length is controlled by capping proteins. Barbed end cappers such as capping protein, gelsolin and tensin block addition of new monomers, so acting to decrease the overall length of the filament. In addition, gelsolin can sever actin filaments, thereby rapidly increasing actin dynamics. A mechanism for gelsolin severing has recently been proposed at the structural level (Burtnick et al., 2004). Depending on the environment in the cell this can have various outcomes, but it is usually a mechanism to disassemble F-actin-containing structures. Pointed end cappers, by contrast, reduce loss of monomer from the pointed end and thereby can lead to rapid filament extension.The best characterised proteins that drive depolymerisation are the actin-depolymerizing factor (ADF) and the cofilin family members. This ubiquitous protein is highly conserved and plays a central role in actin turnover. It binds t