Actin filaments, also known as microfilaments, are thin support filaments produced from chains of the protein actin, which is present in the cells of all eukaryotic organisms. While these filaments serve many different functions, they primarily exist to provide structural support and intracellular transportation as parts of the cellular cytoskeleton. Actin filaments can also play major roles in maintaining or altering cellular shapes and in causing a cell to move. On a larger scale, actin plays an integral part in the process of muscle contraction, without which the actions of humans and many other organisms would be completely impossible. The near ubiquity of actin in cells makes it very useful for a variety of research applications focusing on the cytoskeleton and other areas of cellular biology.
The polymerization of actin, or the process by which monomers of the protein actin combine to form actin filaments, begins with a process called nucleation. Nucleation occurs when a group of three or more actin monomers, spontaneously or otherwise, group together, forming a base onto which other actin monomers can attach. The polymerization of actin does not form a single linear strand; it forms, rather, an actin filament consisting of a double helix of linked actin monomers. Such an arrangement is much more durable than a single linear strand would be.
Actin polymerization is a reversible process, meaning that actin filaments can be broken down into individual units of actin. This makes for a very dynamic process, as actin filaments can polymerize and depolymerize quickly at different locations throughout the cell. Various chemical changes in different parts of a cell can promote polymerization or depolymerization, so actin filaments can be assembled or disassembled quite rapidly based on the particular needs of the cell. There tends to be an apparent dynamic equilibrium between the concentration of actin monomers and filaments, though a variety of factors can affect this equilibrium. Below a certain threshold concentration of monomers, filaments likely will not form, but above that threshold, nucleation and polymerization occur spontaneously.
Actin, because of its near ubiquity in eukaryotic cells and its essential nature as part of the cellular cytoskeleton, is commonly studied in biological experimentation. Various methods have been developed to dye actin so that changes resulting from drugs or genetic modification can be observed. Organisms or cells can be genetically altered or treated with different drugs that affect the polymerization of actin filaments. Such experiments are used to precisely classify the many roles of actin filaments and to learn how altering them affects cells.