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THE CYTOSKELETON
- 2. CYTOSKELETON
- 3. • The cytoskeleton,is the network of protein filaments extending throughout the cytoplasm of all eukaryotic cells • The cytoskeleton provides a structural framework for the cell, that determines cell shape,the positions of organelles, and the general organization of the cytoplasm • The cytoskeleton is composed of three principal types of protein filaments such as Actin filaments, Intermediate filaments, and Microtubules
- 5. FUNCTIONS OF CYTOSKELETON • Establishing cell shape • Providing mechanical strength • Locomotion • Chromosome separation in mitosis and meiosis • Intracellular transport of organelles
- 6. • The major cytoskeletal protein of most cells is Actin • which polymerizes to form actin filaments- thin, flexible fibers approximately 7 nm in diameter and up to several micrometers in length • Being the thinnest of the cytoskeletal filaments,This are also called Microfilaments
- 7. (Actin cytoskeleton stained with phalloidin
- 8. • In skeletal muscle fibers they are called Thin filaments • Generate Cytoplasmic streaming in some cells • Generate Locomotion in cells such as white blood cells • Interact with myosin ("thick") filaments in skeletal muscle fibres to provide the force of muscular contraction
- 10. • Filopodia (also called Microspikes) are long, thin and transient structures that extend out from the cell surface. • Bundles of parallel actin filaments, with their plus ends oriented toward the filopodial tip • Microvilli are shorter and more numerous protrusions of the cell surface found in some cells. • Tightly bundled actin filaments within these structures their plus ends oriented toward the tip • Small cross-linking proteins such as fimbrin and villin bind actin filaments together within microvilli provide stiffness
- 12. Lamellipodia are thin but broad projections at the edge of a mobile cell. Lamellipodia are dynamic structures, constantly changing shape.
- 13. • Stress fibers form when a cell makes stable connections to a substrate • Bundles of actin filaments extend from the cell surface through the cytosol. The actin filaments, whose plus ends are oriented toward the cell surface • Myosin mediates sliding of anti-parallel actin filaments during contraction of stress fibers • -Actinin may cross-link actin filaments within stress fibers
- 15. • Some cells have a cytoskeletal network just inside the plasma membrane that includes actin along with various other proteins such as spectrin • This cytoskeleton has a role in maintaining cell shape. An example is found in erythrocytes • Spectrin is an actin-binding protein that forms an elongated tetrameric complex having an actin-binding domain at each end • With short actin filaments, spectrin forms a cytoskeletal network on the cytosolic surface of the plasma membrane of erythrocytes and some other cells
- 17. ORGANISATION OF ACTIN FILAMENTS • Individual actin filaments are assembled into two general types of structures called Actin bundles and Actin networks, which play different roles in the cell. • The first type of bundle, containing closely spaced actin filaments aligned in parallel supports projections of the plasma membrane, such as microvilli • In these bundles, all the filaments have the same polarity, with their barbed ends adjacent to the plasma membrane • An example of a bundling protein involved in the formation of these structures is fimbrin • which was first isolated from intestinal microvilli and later found in surface projections of a wide variety of cell types
- 19. • The second type of actin bundle is composed of filaments that are more widely spaced, allowing the bundle to contract • In contrast to fimbrin, actin binds to actin as a dimer, each subunit of which is a 102 kd protein containing a single actin-binding site
- 20. • The actin filaments in networks are held together by large actin- binding proteins, such as filamin. • Filamin binds actin as a dimer of two 280 kd subunits. The actin-binding domains and dimerization domains are at opposite ends of each subunit • so the filamin dimer is a flexible V-shaped molecule with actin- binding domains at the ends of each arm. • As a result, filamin forms cross-links between orthogonal actin filaments, creating a loose three-dimensional mesh .
- 22. They have a diameter of about 25 nm They are variable in length but can grow 1000 times as long as they are wide They are built by the assembly of dimers of alpha tubulin and beta tubulin They are straight, hollow cylinders whose wall is made up of a ring of 13 "protofilaments” They are found in both animal and plant cells. In plant cells, microtubules are created at many sites scattered through the cell. In animal cells, the microtubules originate at the centrosome.
- 24. • The attached end is called the minus end; the other end is the plus end • grow at the plus end by the polymerization of tubulin dimers (powered by the hydrolysis of GTP) Microtubules participate in a wide variety of cell activities • Most involve motion. The motion is provided by protein "motors" that use the energy of ATP to move along micotubule
- 26. Microtubule motors • There are two major groups of microtubule motors: • kinesins most of these move toward the plus end of the microtubules • dyneins which move toward the minus end
- 28. Some examples: • The migration of chromosomes in mitosis and meiosis takes place on microtubules that make up the spindle fibres • Both Kinesins and Dyneins are used as motors. • The rapid transport of organelles, like vesicles and mitochondria, along the axons of neurons takes place along microtubules • The motors used are kinesins
- 29. Vincristine, a drug found in the Madagascar periwinkle (a wildflower), binds to tubulin dimers preventing the assembly of microtubules. This halts cells in metaphase of mitosis. Taxol, a drug found in the bark of the Pacific yew, prevents depolymerization of the microtubules of the spindle fiber. This, in turn, stops chromosome movement, and thus prevents the completion of mitosis. • Because the hallmark of cancer cells is uncontrolled mitosis,both vincristine and Taxol are used as anticancer drugs .
- 32. STRUCTURE OF MICROTUBULE • The building blocks of microtubules are tubulin dimers consisting of two closely related 55 kd polypeptides: α- tubulin and β-tubulin • Like actin, both α and β tubulin are encoded by small families of related genes • In addition, a third type of tubulin (y-tubulin) is concentrated in the centrosome where it plays a critical role in initiating microtubule assembly • Tubulin dimers can depolymerize as well as polymerize, and micro-tubules can undergo rapid cycles of assembly and disassembly.
- 34. ASSEMBLY OF MICROTUBULES • In animal cells, most microtubules extend outward from the centrosome • It was first discovered by Theoder Boveri in 1888 • During mitosis Microtubules similarly extend outward from duplicated centrosomes to form mititic spindle • which is responsible for the separation and distribution of chromosomes to daughter cells
- 35. • Microtubules grow by the addition of tubulin to their plus ends • which extend outward from the centrosome toward the cell periphery, so the role of the centrosome is to initiate microtubule growth. • The centrosomes of most animal cells contain a pair of centrioles, oriented perpendicular to each other • The centrioles are cylindrical structures based on nine triplets of microtubules, similar to the basal bodies of cilia and flagella.
- 37. • Cilia and flagella are microtubule-based projections of the plasma membrane that are responsible for movement of a variety of eukaryotic cells • Many bacteria also have flagella, but these prokaryotic flagella are quite different from those of eukaryotes • Bacterial flagella are protein filaments projecting from the cell surface
- 39. • Eukaryotic cilia and flagella are very similar structures, each with a diameter of approximately 0.25 pm • The fundamental structure of both cilia and flagella is the Axoneme,which is composed of microtubules and their associated proteins • The microtubules are arranged in a characteristic "9 + 2" pattern in which a central pair of microtubules is surrounded by nine outer micro- tubule doublets