Overview of Cell Biology/Actin-Binding Proteins
Readings
[edit | edit source]- Study guide:
- Wikipedia article: The Arp2/3 Complex
Actin-Binding Proteins
[edit | edit source]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
[edit | edit source]Monomer Binding Proteins
[edit | edit source]The concentration of G-actin in the cell is about 200 μM (REMEMBER that the Cc of actin is 0.2 μM). Actin is maintainted as a monomer by sequestering proteins such as Thymosin b4 and Profilin.
Nucleating Proteins
[edit | edit source]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
[edit | edit source]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
[edit | edit source]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
[edit | edit source]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
[edit | edit source]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.