Actin filaments are dynamic structures whose growth and disassembly are tightly controlled by additional proteins. These proteins may either promote actin filament nucleation by stabilizing the actin nucleus, catalyze filament elongation or promote actin treadmilling. Some well established proteins that play such roles include the Arp2/3 complex, which facilitates nucleation of filament branches; profilin, which catalyzes ADP to ATP exchange and ADF/cofilin, which mediates filament disassembly. The cooperation between each component is extensive and each element has an optimal concentration.

Factors influencing actin filament length and treadmilling

1. ATP binding on G-actin and free ATP-G-actin concentration

ATP-binding on actin subunits modulates the dynamics of filament assembly, with ATP-binding generally favoring intersubunit interactions and thereby filament assembly [1]. The rate of actin addition to filaments depends on the concentration of free ATP-G-actin whereas the rate of subunit loss does not. At high free ATP-G-actin concentrations the rate of addition exceeds the rate of dissociation and this results in actin filament growth.

2. The rate of ATP-G-actin assembly to the ends

The addition of ATP-G-actin to the two ends of preexisting actin filaments occurs at very different rates (see Table 1) [2]. The addition of free ATP-G-actin at the (-) end is much lower relative to the (+) end.

3. The critical concentration can be adjusted

The (-) and (+) ends have a different criticial concentration (Cc) for actin filament growth. The Cc is defined as the concentration level of free ATP-G-actin where the rate of addition is balanced by the rate of loss and no net growth occurs at that end. At concentrations above the Cc, actin filament growth occurs, wheras below it, there is a loss of subunits and shrinkage occurs. Any protein that alters the Cc, e.g. profilin, will alter actin filament dynamics.

4. Actin binding drugs

Toxins such as phalloidins, cytochalasins, latrunculin A, and jasplakinolide are naturally occurring small molecules that bind to actin and alter its polymerization. Phalloidins inhibit actin filament disassembly by locking adjacent actin subunits together, while cytochalasins bind to the barbed end of actin filaments to prevent actin filament assembly and disassembly at that end (reviewed in [3]). Latrunculin A binds to actin monomers to inhibit their polymerization and thus promotes filament disassembly; latrunculin A may also inhibit nucleotide exchange on actin subunits [4, 5]. Jasplakinolides stabilize actin monomers, thereby enhancing filament nucleation and assembly.

5. Actin binding proteins

Certain actin binding proteins initiate actin filament assembly/disassembly directly, while others influence the ATP binding, the rate of G-actin assembly and the Cc of the filament ends. There are over 60 families of actin-binding proteins (reviewed in [6]) with the main actin monomer-binding proteins in vertebrate cells being thymosin-β4 and profilin. Thymosin-β4 binds strongly to ATP-actin and prevents its assembly into filaments [6]. Because profilin and thymosin-β4 have overlapping binding sites on actin [7, 8], profilin must compete with thymosin-β4 during filament assembly [9]. Other examples of actin-binding proteins include: the nucleators spire and the Arp2/3 complex, elongation factors such as formin, actin cross-linking proteins such as fascin or filamin A, nucleation promoting factors such as WASp, and capping protein.