A thin, helical, single-stranded filament of the cytoskeleton found in the cytoplasm of eukaryotic cells, composed of actin subunits, and functions primarily in maintaining the structural integrity of a cell and cell movements
Cytoskeleton is a cytoplasmic structure composed of protein filaments and microtubules in the cytoplasm, and has a role in controlling cell shape, maintaining intracellular organization, and in cell movement. In eukaryotes, there are three major types of cytoskeleton, namely (1) microfilaments, (2) microtubules, and (3) intermediate filaments. For an overview of the differences between them, see table below.
|type of cytoskeleton||Features||Functions|
|Microfilaments||helical polymer of actin sub-units
Cell locomotion (via filopodia, pseudopodia, or lamellipodia) Intracellular movement or transport Cytokinesis (by aiding centrosomes at opposite poles) Muscle contraction (with myosin filaments) Cytoplasmic streaming
|Microtubules||tubular structure with a diameter of 25nm and length ranging from 200nm to 25μm; exhibits polarity; in cilia and flagella, 9+2 microtubular arrangement
(e.g. alpha-tubulin and beta-tubulin)
Cell locomotion (as axoneme of cilia and flagella) Intracellular transport of organelles (e.g. mitochondria) via dyneins and kinesins Spindle fiber formation
|Intermediate filaments||two anti-parallel helices or dimers of varying protein sub-units with diameters ranging from 8 to 12 nm||Cell shape (by bearing tension)
"Scaffolding" for cell and nucleus Nuclear lamina formation Anchor organelles Cell-cell connections (when with proteins and desmosomes)
The microfilament (also called actin filament) is a helical polymer comprised primarily of actin sub-units, with diameter of 7 nm. Other proteins may also be present and interact with actin and they are called actin-binding protein (ABP). However, the microfilament is largely comprised of actin sub-units, especially the F-actin proteins (which are actin proteins that form a linear polymer filament as opposed to the free G-actin proteins). In vertebrates, actins are of three isoforms, i.e. alpha, beta, and gamma. The beta- and the gamma-actins are the isoforms that exist together in the microfilaments of most cell types. A microfilament is typically comprised of two strands of actin. It is flexible, tough, and has a relatively high tensile strength.
There are generally two types based on structure: bundles and networks. Microfilament bundles are long microfilaments that may associate with contractile proteins (e.g. non-muscular myosin). These microfilaments are involved in moving substances within the cell. Microfilament networks are microfilaments that interlink and connect numerous receptor cells for intercellular communication. A special type of microfilament structure referred to as periodic actin rings occurs in axons. Inside the axons are actin rings that form tetramers with spectrin and together they link the neighboring actin rings. As such, they form a cytoskeleton that supports the axon membrane.
Common biological reactions
Microfilaments assemble from G actin sub-units forming a filament. In a filament, the actin proteins are referred to as F actin. Several actin-binding proteins are involved in the assembly process: motor proteins, branching proteins, capping proteins, etc.
The microfilament provides mechanical support for the cell or maintains structural integrity of the cell by forming a band just beneath the cell membrane. It also participates in certain cell junction by linking transmembrane proteins (e.g., cell surface receptors) to cytoplasmic proteins. It also anchors the centrosomes at opposite poles of the cell during mitosis. It particularly aids in the contraction of the cell during cytokinesis. It is also involved in cytoplasmic streaming (i.e. Intracellular movement, or the flowing of cytoplasm within cells). It enables cell locomotion (through lamellipodia, filopodia, or pseudopodia). It could also interact with myosin ("thick") filaments in skeletal muscle fibers to provide the force of muscular contraction.
- Greek mīkrós ("small") + New Latin fīlāmentum, equivalent to LL fīlā(re), (meaning "to wind thread, spin")
- actin filament
- microfilamentous (adjective)
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