The straight or branched structure formation of actin filaments is controlled by nucleating proteins such as the formins and Arp2/3 complex. Formin-mediated assembly results in straight filaments, whereas Arp2/3 protein complex-mediated assembly results in branched actin filaments.
Arp2/3 complex is a seven-subunit complex consisting of two proteins similar to actin- Arp2 and Arp3, and five other subunits that help keep Arp2 and Arp3 inactive. When required, the complex is activated through conformational changes that occur with the synergistic help of other proteins, such as the regulatory nucleation promoting factors or NPFs, actin filaments, an actin monomer, and ATP. The complex nucleates the formation of a branch or a daughter filament on an existing actin filament. Arp2 and Arp3 have a conserved surface similar to actin and are the first two subunits at the pointed end of the branch. The presence of these subunits promotes the binding of actin monomers to the complex and initiates the formation of the branch.
Formins are homodimers of polypeptides that consist of characteristic domains such as the FH2 domain. FH2 domains associate with each other from head to tail to form a donut-shaped dimer. This dimer nucleates the formation of straight actin filaments at the required site. Formins are responsible for forming the contractile ring that separates daughter cells during cytokinesis and actin filaments bundles in filopodia. As the filament grows, formins are processively or continuously associated with the growing barbed end to protect it from capping proteins. Formins have several isomers that are function-specific in some cases and cannot be interchanged. For example, in fission yeast, three separate isoforms nucleate the formation of actin filaments used in the contractile ring, actin cables, and mating, respectively.
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