Filaments are integral and essential components of many biological processes, from the formation of chromosomes to the creation of new proteins. In addition, filaments play an important role in cell motility, as they are often required for movement through small spaces.
While filaments were once thought to be static structures, it is now known that they are highly dynamic, constantly changing in response to their environment. This flexibility is what allows them to perform such a wide variety of functions within cells.
There are four main types of filaments: actin, intermediate, microtubules, and septins. Each type is composed of different protein subunits that come together to form the filamentous structure. The specific combination of subunits determines the function of the filament.
Actin filaments are perhaps the best-studied type of filament due to their involvement in muscle contraction and cell migration. They are also involved in cytokinesis (the division of one cell into two), chromosome separation during cell division, and maintenance of cell shape. Actin filaments are made up of two different types of subunits: monomers and oligomers. Monomers are individual proteins that can bind to other monomers to form dimers, trimers, tetramers, etc., which make up the backboneof an actin filament. Oligomers are specialized proteins that interact with monomers to regulate assembly/disassembly or cross-linkingof actin filaments (1).
Intermediate filaments (IFs) provide structural support for cells and help resist mechanical stress. There are over 50 different typesof IF proteins that have been identified so far; each type is specific to a particular tissue or cell type (2). Unlike microfilamentsand microtubules, which can rapidly disassemble and reassemble in response to changes in cellular conditions, IFs are much more stableand typically have a longer half-life (3). This stability makes them ideal for supporting cells during periods of high stress or injury. However, recent evidence suggests that some IFs may be capableof undergoing rapid turnover under certain circumstances (4).
Microtubules (MTs) serve several vital functions within cells including organizing chromosomes during cell division(mitosis), transporting vesicles and organelles around cells (cytoskeleton), providing structural support for cells(cytoskeleton), playing a role in cell signaling pathways(5), and contributingtocell motility by helping move ciliaand flagella(6). MTsare composedof tubulinprotein subunits that self-assemble into linear polymers called protofilaments; these protofilaments then further assembleinto hollow cylinders called hollow tubesor microtubuleswith a diameter ~25 nm that make up the MT lattice (7).
Septinsare another type offilamentous proteinthat plays an important role inthe organizationofthe cytoskeleton as well as various other cellular processes such assignalingtransductionpathwaysandviral Replication(8–10). Septinsformpolymer chainsfrommonomericsubunitsthatself-assembleinto higher order structures; howeverthe exact natureofthesestructuresremainspoorly understoodat this time(11). It has been suggestedthatthey may form eitherringor helicalstructuresbut definitive evidencefor eitherof these modelsis lacking(12). Septinshavebeen implicatedinthe controloffissionyeastcell sizeby regulatingcytoplasmicstreamingmovementsthroughtheirinteractionwithmyosinfamilymotorproteinstoforma contractileringaroundthecell periphery13–15