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 actin-binding proteins (ABPs)
A large number of ABPs have been identified ( recently 162 proteins). No doubt more will be identified. Many of the known ABPs bind to the same loci on the surface of actin. A few bind with positive cooperativity and tend to form ternary complexes but rather more bind with negative cooperativity. In myofibrils, at least eight sarcomeric proteins bind to the thin filaments. At least 12 ABPs are membrane-associated proteins, and another nine are membrane receptors or ion transporters. Thirteen ABPs cross-link actin filaments, whereas others enable filaments to interact with other elements of the cytoskeleton. Microfilaments probably do not interact directly with microtubules and/or intermediate filaments but do so via linker proteins.
A large number of ABPs have been identified ( recently 162 proteins). No doubt more will be identified. Many of the known ABPs bind to the same loci on the surface of actin. A few bind with positive cooperativity and tend to form ternary complexes but rather more bind with negative cooperativity. In myofibrils, at least eight sarcomeric proteins bind to the thin filaments. At least 12 ABPs are membrane-associated proteins, and another nine are membrane receptors or ion transporters. Thirteen ABPs cross-link actin filaments, whereas others enable filaments to interact with other elements of the cytoskeleton. Microfilaments probably do not interact directly with microtubules and/or intermediate filaments but do so via linker proteins.
ABPs can be classified into seven groups.
- Monomer-binding proteins sequester G-actin and prevent its polymerization (e.g., thymosin β4, DNase I).
- Filament-depolymerizing proteins induce the conversion of F- to G-actin (e.g., CapZ and cofilin).
- Filament end-binding proteins cap the ends of the actin filament preventing the exchange of monomers at the pointed end (e.g., tropomodulin) and at the barbed end (e.g., CapZ).
- Filament severing proteins shorten the average length of filaments by binding to the side of F-actin and cutting it into two pieces (e.g., gelsolin).
- Cross-linking proteins contain at least two binding sites for F-actin, thus facilitating the formation of filament bundles, branching filaments, and three-dimensional networks (e.g., Arp2/3).
- Stabilizing proteins bind to the sides of actin filaments and prevent depolymerization (e.g., tropomyosin).
- Motor proteins that use F-actin as a track upon which to move (e.g., the myosin family of motors).
ABPs are not limited to one class, for example, gelsolin is capable of severing and capping the barbed end of actin filaments, and the Arp2/3 complex can nucleate filament formation, elongate filaments, and establish branch points in actin networks
Refferences
http://physrev.physiology.org
http://jcs.biologists.org
Actin Binding Protein (ABP)