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The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is composed of similar proteins in the various organisms. It is composed of three main components: microfilaments, intermediate filaments, and microtubules, and these are all capable of rapid growth or disassembly depending on the cell's requirements.
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.
The nucleoplasm, also known as karyoplasm, is the type of protoplasm that makes up the cell nucleus, the most prominent organelle of the eukaryotic cell. It is enclosed by the nuclear envelope, also known as the nuclear membrane. The nucleoplasm resembles the cytoplasm of a eukaryotic cell in that it is a gel-like substance found within a membrane, although the nucleoplasm only fills out the space in the nucleus and has its own unique functions. The nucleoplasm suspends structures within the nucleus that are not membrane-bound and is responsible for maintaining the shape of the nucleus. The structures suspended in the nucleoplasm include chromosomes, various proteins, nuclear bodies, the nucleolus, nucleoporins, nucleotides, and nuclear speckles.
Telophase is the final stage in both meiosis and mitosis in a eukaryotic cell. During telophase, the effects of prophase and prometaphase are reversed. As chromosomes reach the cell poles, a nuclear envelope is re-assembled around each set of chromatids, the nucleoli reappear, and chromosomes begin to decondense back into the expanded chromatin that is present during interphase. The mitotic spindle is disassembled and remaining spindle microtubules are depolymerized. Telophase accounts for approximately 2% of the cell cycle's duration.
Intermediate filaments (IFs) are cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate Branchiostoma.
Hemidesmosomes are very small stud-like structures found in keratinocytes of the epidermis of skin that attach to the extracellular matrix. They are similar in form to desmosomes when visualized by electron microscopy, however, desmosomes attach to adjacent cells. Hemidesmosomes are also comparable to focal adhesions, as they both attach cells to the extracellular matrix. Instead of desmogleins and desmocollins in the extracellular space, hemidesmosomes utilize integrins. Hemidesmosomes are found in epithelial cells connecting the basal epithelial cells to the lamina lucida, which is part of the basal lamina. Hemidesmosomes are also involved in signaling pathways, such as keratinocyte migration or carcinoma cell intrusion.
Enaptin also known as nesprin-1 or synaptic nuclear envelope protein 1 (syne-1) is an actin-binding protein that in humans that is encoded by the SYNE1 gene.
Spectrin is a cytoskeletal protein that lines the intracellular side of the plasma membrane in eukaryotic cells. Spectrin forms pentagonal or hexagonal arrangements, forming a scaffold and playing an important role in maintenance of plasma membrane integrity and cytoskeletal structure. The hexagonal arrangements are formed by tetramers of spectrin subunits associating with short actin filaments at either end of the tetramer. These short actin filaments act as junctional complexes allowing the formation of the hexagonal mesh. The protein is named spectrin since it was first isolated as a major protein component of human red blood cells which had been treated with mild detergents; the detergents lysed the cells and the hemoglobin and other cytoplasmic components were washed out. In the light microscope the basic shape of the red blood cell could still be seen as the spectrin-containing submembranous cytoskeleton preserved the shape of the cell in outline. This became known as a red blood cell "ghost" (spectre), and so the major protein of the ghost was named spectrin.
Plectin is a giant protein found in nearly all mammalian cells which acts as a link between the three main components of the cytoskeleton: actin microfilaments, microtubules and intermediate filaments. In addition, plectin links the cytoskeleton to junctions found in the plasma membrane that structurally connect different cells. By holding these different networks together, plectin plays an important role in maintaining the mechanical integrity and viscoelastic properties of tissues.
The cell cortex, also known as the actin cortex, cortical cytoskeleton or actomyosin cortex, is a specialized layer of cytoplasmic proteins on the inner face of the cell membrane. It functions as a modulator of membrane behavior and cell surface properties. In most eukaryotic cells lacking a cell wall, the cortex is an actin-rich network consisting of F-actin filaments, myosin motors, and actin-binding proteins. The actomyosin cortex is attached to the cell membrane via membrane-anchoring proteins called ERM proteins that plays a central role in cell shape control. The protein constituents of the cortex undergo rapid turnover, making the cortex both mechanically rigid and highly plastic, two properties essential to its function. In most cases, the cortex is in the range of 100 to 1000 nanometers thick.
In biology, a protein filament is a long chain of protein monomers, such as those found in hair, muscle, or in flagella. Protein filaments form together to make the cytoskeleton of the cell. They are often bundled together to provide support, strength, and rigidity to the cell. When the filaments are packed up together, they are able to form three different cellular parts. The three major classes of protein filaments that make up the cytoskeleton include: actin filaments, microtubules and intermediate filaments.
Emerin is a protein that in humans is encoded by the EMD gene, also known as the STA gene. Emerin, together with LEMD3, is a LEM domain-containing integral protein of the inner nuclear membrane in vertebrates. Emerin is highly expressed in cardiac and skeletal muscle. In cardiac muscle, emerin localizes to adherens junctions within intercalated discs where it appears to function in mechanotransduction of cellular strain and in beta-catenin signaling. Mutations in emerin cause X-linked recessive Emery–Dreifuss muscular dystrophy, cardiac conduction abnormalities and dilated cardiomyopathy.
KASH domains are conserved C-terminal protein regions less than ~30 amino acids. KASH is an acronym for Klarsicht, ANC-1, Syne Homology. KASH domains always follow a transmembrane domain. Most proteins containing KASH domains are thought to be involved in the positioning of the nucleus in the cell. KASH domains interact with proteins containing SUN domains in the space between the outer and inner nuclear membranes to bridge the nuclear envelope, and may transfer force from the nucleoskeleton to the cytoplasmic cytoskeleton and enable mechanosensory roles in cells. KASH proteins are thought to largely localize to the outer nuclear membrane, although there are reports of inner nuclear membrane localization of some KASH protein isoforms.
The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material.
Dynactin is a 23 subunit protein complex that acts as a co-factor for the microtubule motor cytoplasmic dynein-1. It is built around a short filament of actin related protein-1 (Arp1).
Alpha II-spectrin, also known as Spectrin alpha chain, brain is a protein that in humans is encoded by the SPTAN1 gene. Alpha II-spectrin is expressed in a variety of tissues, and is highly expressed in cardiac muscle at Z-disc structures, costameres and at the sarcolemma membrane. Mutations in alpha II-spectrin have been associated with early infantile epileptic encephalopathy-5, and alpha II-spectrin may be a valuable biomarker for Guillain–Barré syndrome and infantile congenital heart disease.
Alpha-centractin (yeast) or ARP1 is a protein that in humans is encoded by the ACTR1A gene.
Nesprin-2 is a protein that in humans is encoded by the SYNE2 gene. The human SYNE2 gene consists of 116 exons and encodes nesprin-2, a member of the nuclear envelope (NE) spectrin-repeat (nesprin) family. Nesprins are modular proteins with a central extended spectrin-repeat (SR) rod domain and a C-terminal Klarsicht/ANC-1/Syne homology (KASH) transmembrane domain, which acts as a NE-targeting motif. Nesprin-2 (Nesp2) binds to cytoplasmic F-actin, tethering the nucleus to the cytoskeleton and maintaining the structural integrity of the nucleus.
The LINC complex is a protein complex associated with both inner and outer membranes of the nucleus. It is composed of SUN-domain proteins and KASH-domain proteins. The SUN-domain proteins are associated with both nuclear lamins and chromatin and cross the inner nuclear membrane. They interact with the KASH domain proteins in the perinuclear (lumen) space between the two membranes. The KASH domain proteins cross the outer nuclear membrane and interact with actin filaments, microtubule filaments, intermediate filaments, centrosomes and cytoplasmic organelles. The number of SUN-domain and KASH-domain proteins increased in evolution.
Spectrin repeat containing nuclear envelope family member 3, or Nesprin-3, is a Nesprin-family protein that in humans is encoded by the SYNE3 gene. Nesprin-3 localizes to the outer nuclear membrane, where it is retained by SUN domain-containing proteins. The n-terminus of Nesprin-3 faces the cytoplasm and associates with the cytolinker protein Plectin.
References
- ↑ Zhang Q, Skepper JN, Yang F, Davies JD, Hegyi L, Roberts RG, Weissberg PL, Ellis JA, Shanahan CM (December 2001). "Nesprins: a novel family of spectrin-repeat-containing proteins that localize to the nuclear membrane in multiple tissues". Journal of Cell Science. 114 (Pt 24): 4485–98. doi:10.1242/jcs.114.24.4485. PMID 11792814.
- 1 2 3 Rajgor D, Shanahan CM (July 2013). "Nesprins: from the nuclear envelope and beyond". Expert Reviews in Molecular Medicine. 15: e5. doi:10.1017/erm.2013.6. PMC 3733404 . PMID 23830188.
- ↑ Wilhelmsen K, Ketema M, Truong H, Sonnenberg A (December 2006). "KASH-domain proteins in nuclear migration, anchorage and other processes". Journal of Cell Science. 119 (Pt 24): 5021–9. doi: 10.1242/jcs.03295 . PMID 17158909.
- ↑ Wilhelmsen K, Litjens SH, Kuikman I, Tshimbalanga N, Janssen H, van den Bout I, Raymond K, Sonnenberg A (December 2005). "Nesprin-3, a novel outer nuclear membrane protein, associates with the cytoskeletal linker protein plectin". The Journal of Cell Biology. 171 (5): 799–810. doi:10.1083/jcb.200506083. PMC 2171291 . PMID 16330710.
- ↑ Roux KJ, Crisp ML, Liu Q, Kim D, Kozlov S, Stewart CL, Burke B (February 2009). "Nesprin 4 is an outer nuclear membrane protein that can induce kinesin-mediated cell polarization". Proceedings of the National Academy of Sciences of the United States of America. 106 (7): 2194–9. Bibcode:2009PNAS..106.2194R. doi: 10.1073/pnas.0808602106 . PMC 2650131 . PMID 19164528.
- ↑ Uzer G, Thompson WR, Sen B, Xie Z, Yen SS, Miller S, et al. (June 2015). "Cell Mechanosensitivity to Extremely Low-Magnitude Signals Is Enabled by a LINCed Nucleus". Stem Cells. 33 (6): 2063–76. doi:10.1002/stem.2004. PMC 4458857 . PMID 25787126.
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