Aktinpolymerisation

Aktinpolymerisation is the process by which globular actin (G-actin) monomers assemble into filamentous actin (F-actin). In eukaryotic cells, actin polymerisation is ATP-dependent: G-actin binds ATP, incorporates into filaments, and ATP is hydrolysed after incorporation, influencing filament stability and turnover.

The assembly proceeds through nucleation, elongation, and steady state. Nucleation is rate-limiting and uses actin-nucleating factors such as the Arp2/3 complex or formins. During elongation, monomers add mainly to the barbed (+) end, which grows faster than the pointed (-) end. Two ends have different critical concentrations, and in networks filaments can treadmill with subunits adding at the plus end and dissociating at the minus end.

Actin dynamics are regulated by binding proteins. Profilin delivers ATP-actin to growing filaments; thymosin beta-4 sequesters G-actin; cofilin promotes disassembly of aged filaments. Capping proteins limit growth; Arp2/3 and formins control nucleation; tropomyosin stabilizes filaments; cross-linking proteins organize networks into branched or bundled structures.

Functions of actin polymerisation include cell migration, maintenance of cell shape, cytokinesis, endocytosis, and vesicle trafficking. Actin networks form lamellipodia and filopodia in migrating cells and provide tracks for myosin motors in muscle and non-muscle contexts, contributing to force generation and intracellular transport.

Research uses pharmacological agents that perturb actin dynamics to study cellular processes. Dysregulation of actin polymerisation is linked to diseases affecting cell movement, adhesion, and tissue organization, making it a central topic in cell biology.