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Cell surface (cortical) actin remodeling is a cyclic (9-step) process where each step is directly responsive to a cell signaling mechanism. Over the course of the cycle, actin begins as a monomer, elongates into a polymer with the help of attached actin-binding-proteins, and disassembles back into a monomer so the remodeling cycle may commence again.
Many actin-related molecules create a free barbed end for polymerization by uncapping or severing pre-existing filaments and using these as actin nucleation cores. However, the Arp2/3 complex stimulates actin polymerization by creating a new nucleation core. Actin nucleation is an initial step in the formation of an actin filament.
Actin plays a role in the formation of new spines as well as stabilizing spine volume increase. [1] The changes that actin brings about lead to the formation of new synapses as well as increased cell communication. Actin remodeling consists of the dynamic changes in actin polymerization that underlie the morphological changes at the neural synapse.
The following steps describe one force-generating cycle of an actoclampin molecular motor: The polymerization cofactor profilin and the ATP·actin combine to form a profilin-ATP-actin complex that then binds to the end-tracking unit; The cofactor and monomer are transferred to the barbed-end of an actin already clamped filament
Association of G-actin into F-actin is regulated by the critical concentration outlined below. Actin polymerization can further be regulated by profilin and cofilin. [6] Cofilin functions by binding to ADP-actin on the negative end of the filament, destabilizing it, and inducing depolymerization.
Older" ADP/ADP-Pi actin filaments free of tropomyosin and proper pH are required for cofilin to function effectively. In the presence of readily available ATP-G-actin cofilin speeds up actin polymerization via its actin-severing activity (providing free barbed ends for further polymerization and nucleation by the Arp2/3 complex). [13]
Actin-binding proteins (also known as ABPs) are proteins that bind to actin. [1] This may mean ability to bind actin monomers, or polymers, or both. Many actin-binding proteins, including α-actinin, β-spectrin, dystrophin, utrophin and fimbrin, do this through the actin-binding calponin homology domain .
The unfavorable kinetics of actin oligomer production prevent spontaneous actin polymerization. [2] Once an actin nucleus has been created, the connection of the monomers happens swiftly, with the plus end developing considerably more quickly than the minus end. [2] Actin's ATPase activity sharply rises after insertion into the filament. [2]