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Their formation and turnover are regulated by many proteins, including: [citation needed] Filament end-tracking protein (e.g., formins, VASP, N-WASP) Filament-nucleator known as the Actin-Related Protein-2/3 (or Arp2/3) complex; Filament cross-linkers (e.g., α-actinin, fascin, and fimbrin) Actin monomer-binding proteins profilin and thymosin β4
Microfilament Polymerization. Microfilament polymerization is divided into three steps. The nucleation step is the first step, and it is the rate limiting and slowest step of the process. Elongation is the next step in this process, and it is the rapid addition of actin monomers at both the plus and minus end of the microfilament.
Eukaryotic cells contain three main kinds of cytoskeletal filaments: microfilaments, microtubules, and intermediate filaments. In neurons the intermediate filaments are known as neurofilaments. [16] Each type is formed by the polymerization of a distinct type of protein subunit and has its own characteristic shape and intracellular distribution.
The protein gelsolin, which is a key regulator in the assembly and disassembly of actin. Other proteins bind to the ends of actin filaments, stabilizing them. These are called "capping proteins" and include CapZ and tropomodulin. CapZ binds the (+) end of a filament, preventing further addition or loss of actin from that end. [121]
The oligomerization is the rate-determining step, considering actin filament formation as a whole. The so-called lag phase of actin polymerization originates from this step. It takes quite a while until polymerization starts, but once it has, the process is autocatalytic until the physiological maximum of the polymerization rate is reached.
The use of cytochalasins has allowed researchers to better understand actin polymerization, cell motility, ruffling, cell division, contraction, and cell stiffness. The use of cytochalasins has been so important to understanding cytoskeletal movement and many other biological processes, researchers have created two synthetic cytochalasins.
Within the lamellipodia are ribs of actin called microspikes, which, when they spread beyond the lamellipodium frontier, are called filopodia. [2] The lamellipodium is born of actin nucleation in the plasma membrane of the cell [1] and is the primary area of actin incorporation or microfilament formation of the cell.
The C protein plays an important role in the retrograde transport of vesicles and is also known as cytoplasmic dynein. MAP-2 proteins are located in the dendrites and in the body of neurons, where they bind with other cytoskeletal filaments. The MAP-4 proteins are found in the majority of cells and stabilize microtubules.