aktinpolymerisering

Aktinpolymerisering is the process by which actin monomers (G-actin) assemble into filamentous actin (F-actin) in eukaryotic cells. This dynamic process is essential for cell shape, motility, division and intracellular transport. Polymerization is ATP-dependent: G-actin binds ATP and adds to growing filaments; after incorporation, the bound ATP is hydrolyzed to ADP, which influences filament stability.

Polymerization proceeds through nucleation, elongation and steady state. Nucleation, the rate-limiting step, is catalyzed by actin nucleators such as formins (leading to unbranched filaments) and the Arp2/3 complex (creating branched networks). Elongation occurs mainly at the barbed (+) end, while the pointed (-) end grows more slowly. The end-specific kinetics give rise to different critical concentrations, and at intermediate G-actin concentrations the system can exhibit treadmilling, in which subunits add at the barbed end and dissociate at the pointed end, yielding turnover without net filament growth.

Regulation is achieved by a variety of actin-binding proteins (ABPs). Profilin delivers ATP–G-actin to barbed ends, thymosin beta-4 sequesters monomers, and capping proteins terminate growth at ends. Cofilin promotes disassembly and filament turnover, while gelsolin can sever filaments. Cross-linking proteins such as alpha-actinin organize filaments into networks and bundles, supporting lamellipodia, filopodia and stress fibers.

Research methods include in vitro polymerization assays (e.g., pyrene-actin fluorescence) and live-cell imaging to study dynamics. Proper actin polymerization is crucial for many cellular processes, and dysregulation is implicated in diseases ranging from cancer metastasis to neurodegenerative disorders.