Microfilaments can be assembled and disassembled. When ATP is combined with monomers, there will be high mutual affinity, and monomers tend to polymerize into polymers, that is, assembly. However, when ATP is hydrolyzed into ADP, the affinity of monomer will decrease, and the polymer will tend to depolymerize, that is, disassemble. High ATP concentration is beneficial to the assembly of microfilament. Therefore, when the cytoplasm is put into ATP-rich solution, it will quickly solidify into gel due to the massive assembly of microfilaments. The assembly speed of both ends of microfilament is different. The fast end (+pole) is 5 to 10 times faster than the slow end (-pole). When the concentration of ATP reaches a certain critical value, the phenomenon of+pole assembly and-pole disassembly can be observed at the same time, which is called "trampling behavior" The assembly of microfilaments can be divided into three stages: nucleation stage, growth stage or extension stage and equilibrium stage. Nucleation period is the rate-limiting process of microfilament assembly, which takes some time, so it is also called delay period. At this time, myosin begins to polymerize, and its dimer is unstable and easy to hydrolyze, and it is stable only when trimer is formed, that is, nucleation. Once the core is formed, globular myosin quickly polymerizes at both ends of the core and enters the growth period. The assembly speed of the two ends of the microfilament is different, and the assembly speed of the positive end is obviously faster than that of the negative end, which is about 10 times of that of the negative end. When the microfilament is elongated for a certain period, the speed of actin incorporation into the microfilament and dissociation from the negative end of the microfilament reaches a balance. At this time, the equilibrium period is entered, the length of microfilament is basically unchanged, the length of positive end is equal to that of negative end, and the polymerization and dissociation activities are still going on.
The assembly of microfibers can be explained by trampling model and dynamic instability model, but the latter is more reasonable. ATP is the main factor to regulate the dynamic unstable behavior of microfilament assembly. In addition, actin binding protein (ABP) can also regulate the assembly of microfilaments. The assembly and disassembly of microfilaments are regulated by many protein in cytoplasm. These protein can bind to microfilaments and affect the assembly and disassembly speed of microfilaments, which is called binding protein.
The assembly of microfilaments requires "nucleation" first, that is, several monomers are polymerized first, and other monomers are combined with them to form a larger polymer. Arp complex (Actin related-protein) is a kind of protein which can bind to actin, and it can be used as a template to promote the polymerization of actin. The Arp complex consists of Arp2, Arp3 and five other protein.
Terminal blocking protein is the "hat" at both ends of microfilament. When this protein is combined with microfilament, the assembly and disassembly of microfilament will stop. This is very important for some protein with fixed length, such as filaments.
Prefibrillin (or translated G actin binding protein) promotes polymerization, and correspondingly, the protein that promotes depolymerization is fibrillin. Filament cutting proteins, such as gelsolin, can cut microfilaments from the middle. Neurin can fix microfilaments on the cell membrane and form attachment points. Cross-linked proteins have more than two actin binding sites, which play the role of connecting microfilaments. Among them, fimbrin helps the microfilaments to form bundles, while filamin cross-links the microfilaments into a network.