Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other proteins in the cell. Microfilaments are usually about 7 nm in diameter and made up of two strands of actin. Microfilament functions include cytokinesis, amoeboid movement, cell motility, changes in cell shape, endocytosis and exocytosis, cell contractility, and mechanical stability. Microfilaments are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces. In inducing cell motility, one end of the actin filament elongates while the other end contracts, presumably by myosin II molecular motors. Additionally, they function as part of actomyosin-driven contractile molecular motors, wherein the thin filaments serve as tensile platforms for myosin's ATP-dependent pulling action in muscle contraction and pseudopod advancement. Microfilaments have a tough, flexible framework which helps the cell in movement.
Actin is a family of globular multi-functional proteins that form microfilaments. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over 100 μM; its mass is roughly 42-kDa, with a diameter of 4 to 7 nm.
Tropomyosin is a two-stranded alpha-helical coiled coil protein found in actin-based cytoskeletons.
Actin-binding protein are proteins that bind to actin. This may mean ability to bind actin monomers, or polymers, or both.
ADF/cofilin is a family of actin-binding proteins associated with the rapid depolymerization of actin microfilaments that give actin its characteristic dynamic instability. This dynamic instability is central to actin's role in muscle contraction, cell motility and transcription regulation.
Major sperm protein (MSP) is a nematode specific small protein of 126 amino acids with a molecular weight of 14 kDa. It is the key player in the motility machinery of nematodes that propels the crawling movement/motility of nematode sperm. It is the most abundant protein present in nematode sperm, comprising 15% of the total protein and more than 40% of the soluble protein. MSP is exclusively synthesized in spermatocytes of the nematodes. The MSP has two main functions in the reproduction of the helminthes: i) as cytosolic component it is responsible for the crawling movement of the mature sperm, and ii) once released, it acts as hormone on the female germ cells, where it triggers oocyte maturation and stimulates the oviduct wall to contract to bring the oocytes into position for fertilization. MSP has first been identified in Caenorhabditis elegans.
Gelsolin is an actin-binding protein that is a key regulator of actin filament assembly and disassembly. Gelsolin is one of the most potent members of the actin-severing gelsolin/villin superfamily, as it severs with nearly 100% efficiency.
Cofilin 1 , also known as CFL1, is a human gene, part of the ADF/cofilin family.
A DNA clamp, also known as a sliding clamp or β-clamp, is a protein complex that serves as a processivity-promoting factor in DNA replication. As a critical component of the DNA polymerase III holoenzyme, the clamp protein binds DNA polymerase and prevents this enzyme from dissociating from the template DNA strand. The clamp-polymerase protein–protein interactions are stronger and more specific than the direct interactions between the polymerase and the template DNA strand; because one of the rate-limiting steps in the DNA synthesis reaction is the association of the polymerase with the DNA template, the presence of the sliding clamp dramatically increases the number of nucleotides that the polymerase can add to the growing strand per association event. The presence of the DNA clamp can increase the rate of DNA synthesis up to 1,000-fold compared with a nonprocessive polymerase.
Glia maturation factor is a neurotrophic factor implicated in nervous system development, angiogenesis and immune function. In humans, the glia maturation factor beta and glia maturation factor gamma proteins are encoded by the GMFB and GMFG genes, respectively.
Destrin or DSTN is a protein which in humans is encoded by the DSTN gene. Destrin is a component protein in microfilaments.
Twinfilin-1 is a protein that in humans is encoded by the TWF1 gene. This gene encodes twinfilin, an actin monomer-binding protein conserved from yeast to mammals. Studies of the mouse counterpart suggest that this protein may be an actin monomer-binding protein, and its localization to cortical G-actin-rich structures may be regulated by the small GTPase RAC1.
Cofilin 2 (muscle) also known as CFL2 is a protein which in humans is encoded by the CFL2 gene.
Protein cordon-bleu is a protein that in humans is encoded by the COBL gene.
The Actin assembly-inducing protein (ActA) is a protein encoded and used by Listeria monocytogenes to propel itself through a mammalian host cell. ActA is a bacterial surface protein comprising a membrane-spanning region. In a mammalian cell the bacterial ActA interacts with the Arp2/3 complex and actin monomers to induce actin polymerization on the bacterial surface generating an actin comet tail. The gene encoding ActA is named actA or prtB.
Actin remodeling is the biochemical process that allows for the dynamic alterations of cellular organization. The remodeling of actin filaments occurs in a cyclic pattern on cell surfaces and exists as a fundamental aspect to cellular life. During the remodeling process, actin monomers polymerize in response to signaling cascades that stem from environmental cues. The cell's signaling pathways cause actin to affect intracellular organization of the cytoskeleton and often consequently, the cell membrane. Again triggered by environmental conditions, actin filaments break back down into monomers and the cycle is completed. Actin-binding proteins (ABPs) aid in the transformation of actin filaments throughout the actin remodeling process. These proteins account for the diverse structure and changes in shape of Eukaryotic cells. Despite its complexity, actin remodeling may result in complete cytoskeletal reorganization in under a minute.
mDia1 is a member of the protein family called the formins and is a Rho effector. It is the mouse version of the diaphanous homolog 1 of Drosophila. mDia1 localizes to cells' mitotic spindle and midbody, plays a role in stress fiber and filopodia formation, phagocytosis, activation of serum response factor, formation of adherens junctions, and it can act as a transcription factor. mDia1 accelerates actin nucleation and elongation by interacting with barbed ends of actin filaments. The gene encoding mDia1 is located on Chromosome 18 of Mus musculus and named Diap1.
Beta thymosins are a family of proteins which have in common a sequence of about 40 amino acids similar to the small protein thymosin β4. They are found almost exclusively in multicellular animals. Thymosin β4 was originally obtained from the thymus in company with several other small proteins which although named collectively "thymosins" are now known to be structurally and genetically unrelated and present in many different animal tissues.
In molecular biology, the cyclase-associated protein family (CAP) is a family of highly conserved actin-binding proteins present in a wide range of organisms including yeast, flies, plants, and mammals. CAPs are multifunctional proteins that contain several structural domains. CAP is involved in species-specific signalling pathways. In Drosophila, CAP functions in Hedgehog-mediated eye development and in establishing oocyte polarity. In Dictyostelium discoideum, CAP is involved in microfilament reorganisation near the plasma membrane in a PIP2-regulated manner and is required to perpetuate the cAMP relay signal to organise fruitbody formation. In plants, CAP is involved in plant signalling pathways required for co-ordinated organ expansion. In yeast, CAP is involved in adenylate cyclase activation, as well as in vesicle trafficking and endocytosis. In both yeast and mammals, CAPs appear to be involved in recycling G-actin monomers from ADF/cofilins for subsequent rounds of filament assembly. In mammals, there are two different CAPs that share 64% amino acid identity.
