Overview of Cell Biology/Actin-Binding Proteins

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• Wikipedia article: The Arp2/3 Complex

Actin-Binding Proteins

Assembly of pure actin in vitro is different from cellular actin. F-actin is depolymerized by dilution in a low salt buffer.

Functions of Actin-Binding Proteins

Monomer Binding Proteins

The concentration of G-actin in the cell is about 200 μM (REMEMBER that the C_c of actin is 0.2 μM). Actin is maintainted as a monomer by sequestering proteins such as Thymosin b4 and Profilin.

Nucleating Proteins

Nucleating proteins promote polymerization and are usually present in areas that undergo dynamic extension/retration. One key nucleating factor in cells is the Arp2/3 Complex which works in conjuction with the nucleation proting factor family of WASPs. The Arp2/3 and WASP nucleating activity form a filament that is capped at the (-) end of the filament. Another family of nucleating factors are formins, which nucleate in a way that leaves both ends of the polymer free to bind new monomers.

Capping Proteins

Capping proteins stabilize filaments by capping to one end and preventing the addition or loss of new subunits.

Examples:

- CapZ caps at the (+) end.

- Tropomodulin binds at the (-) end.

- Tropomyosin binds along the length of the filament.

Severing and Depolymerizing Proteins

A protein called ADF-cofilin binds to ADP associated monomers and then severing filaments and acclerating depolymerization of ADP actin from the minus ends. ADF-cofilin proteins are regulated in cells by phosphorylatin via LIM kinase.

During ameboid motility, cytosol flos from the center of the amoeba and turns into a gel when it reaches the front of the cell. The gel is turned to sol by gelsolin which severs actin filaments. After being severing, gelsolin remains bound to the (+) end of the filament. Gelsolin is inactive when Ca²⁺ concentrations in the cytoplasm are low

Networking and Bundling Proteins

Proteins with shorter spacers tend to bundle filaments into parallel arrays. Crosslinking proteins tend to have longer spacers and arrange filaments in networks.

Examples of bundling proteins: fascin, villin, fimbrin, α-actinin

Examples of crosslinking proteins: spectrin, dystrophin, filamin

Cross-linking at the Plasma Membrane

In muscle cells, cytoskeleton is linked to the membrane by dystrophin. Dystrophin was discovered as the gene mytated in Duchenne muscular dystrophy. Dystrophin binds to a membrane glycoprotein complex and stabilizes muscle plasma membrane.