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Cytoskeleton

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Cytoskeleton

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Cytoskeleton

  1. 1. The cytoskeleton
  2. 2. The cytoskeleton is a network of fibers that organizes structures and activities in the cell • The cytoskeleton is a network of fibers extending throughout the cytoplasm • It organizes the cell’s structures and activities, anchoring many organelles • It is composed of three types of molecular structures: – Microtubules – Microfilaments – Intermediate filaments Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  3. 3. Fig. 6-20 Microtubule 0.25 μm Microfilaments
  4. 4. Roles of the Cytoskeleton: Support, Motility, and Regulation • The cytoskeleton helps to support the cell and maintain its shape • It interacts with motor proteins to produce motility • Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton • Recent evidence suggests that the cytoskeleton may help regulate biochemical activities Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  5. 5. Fig. 6-21 Vesicle ATP Receptor for motor protein Microtubule of cytoskeleton Motor protein (ATP powered) (a) Microtubule Vesicles (b) 0.25 μm
  6. 6. Components of the Cytoskeleton • Three main types of fibers make up the cytoskeleton: – Microtubules are the thickest of the three components of the cytoskeleton – Microfilaments, also called actin filaments, are the thinnest components – Intermediate filaments are fibers with diameters in a middle range Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  7. 7. Table 6-1 10 μm 10 μm 10 μm Column of tubulin dimers Tubulin dimer Actin subunit a b 25 nm 7 nm Keratin proteins Fibrous subunit (keratins coiled together) 8–12 nm
  8. 8. Table 6-1a 10 μm Column of tubulin dimers Tubulin dimer a b 25 nm
  9. 9. Table 6-1b Actin subunit 10 μm 7 nm
  10. 10. Table 6-1c 5 μm Keratin proteins Fibrous subunit (keratins coiled together) 8–12 nm
  11. 11. Cytoskeleton proteins revealed by Commassie staining Cytoskeleton: filament system Three types of filaments and accessory proteins (assembly of cytoskeleton, motorproteins that move organelles or filaments) Internal order Shape and remodel surface Move organelles Movement Cell division
  12. 12. Dynamic and adaptable Actin filaments: shape of the cell’s surface and whole cell locomotion 5-9 nm diameter Microtubules: positions of membrane-enclosed organelles, intracellular transport 25 nm diameter Intermediate filaments: mechanical strength and resistance to shear stress 10 nm diameter
  13. 13. Microtubules • Microtubules are hollow rods about 25 nm in diameter and about 200 nm to 25 microns long • Functions of microtubules: – Shaping the cell – Guiding movement of organelles – Separating chromosomes during cell division Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  14. 14. Centrosomes and Centrioles • In many cells, microtubules grow out from a centrosome near the nucleus • The centrosome is a “microtubule-organizing center” • In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  15. 15. Fig. 6-UN3
  16. 16. Microtubules • Microtubules • I. Introduction: Long, hollow cylinders, 25 nm in diameter, made of tubulin. The basic subunit is a heterodimer of α and β tubulin (9.8); 13 protofilaments in a typical cylinder. See below about GTP binding, treadmilling, growth and dynamic instability (9.26). There is a + end, fast growing, w/β tubulin at its end, and a – end, slow growing, w/α tubulin at its end. The GTP’s are important in assembly (9.8) • A. They have MAPs, that influence their use- linking them together, stabilizing them, or destabilizing them. • B. They form a network, coming from the microtubule organizing center, which is usually the centrosome or centriole, w/ the – end anchored there. (9.10-13, 19) • C. Also form cilia and flagella, and spindle fibers in mitosis.
  17. 17. 13 protofilaments
  18. 18. • F. MICROTUBULE DYNAMICS: 9.25 • Key points- the cap means that subunits are added easily- loss of GTP = harder to add subunits, need higher subunit conc. to add. • Produces microtubule catastrophes!
  19. 19. Nucleation • Gamma tubulin in MTOC/centriole- MT’s grow from there
  20. 20. FIGURE 9.25 The structural cap model of dynamic instability. According to the model, the growth or shrinkage of a microtubule depends on the state of the tubulin dimers at the plus end of the microtubule. Tubulin-GTP dimers are depicted in red. Tubulin-GDP dimers are depicted in blue. In a growing microtubule (step 1), the tip consists of an open sheet containing tubulin-GTP subunits. In step 2, the tube has begun to close, forcing the hydrolysis of the bound GTP. In step 3, the tube has closed to its end, leaving only tubulin- GDP subunits. GDP-tubulin subunits have a curved conformation compared to their GTP-bound counterparts, which makes them less able to fit into a straight protofilament. The strain resulting from the presence of GDP-tubulin subunits at the plus end of the microtubule is released as the protofilaments curl outward from the tubule and undergo catastrophic shrinkage (step 4).
  21. 21. Dynamic instability:predominant in microtubules GTP hydrolysis “catch up” Treadmilling: predominant in actin filaments
  22. 22. Lateral bonds force GDP-containing protofilaments into a linear conformation
  23. 23. Remodeling • The fact that MT’s aren’t fixed means that cells can remodel their shape- plant cells, our cells in mitosis- round up, as MT’s used to make spindle fibers
  24. 24. The time course of actin polymerization in a test tube
  25. 25. The structure of a microtubule and its subunits GTP! GTP hollow and cylindrica and polar 13 parallel protofilaments heterodimer
  26. 26. The structure of an actin monomer and actin filament monomer ATP polar two parallel protofilaments that twist around each other in a right-handed helix Flexible but cross-linked and bundled together by accessory proteins in a living cell
  27. 27. The preferential growth of microtubules at the plus end Plus end: polymerize and depolymerize faster than minus end Microtubules: Plus end- b subunit Minus end- a subunit Actin filaments Plus end- barbed end Minus end- pointed end
  28. 28. Fig. 6-22 Centrosome Microtubule Centrioles 0.25 μm Longitudinal section of one centriole Microtubules Cross section of the other centriole
  29. 29. GTP hydrolysis causes filament to curve
  30. 30. MT drugs • Colchicine- prevents MT formation- arrests cells at metaphase • Useful in determining role of MT’s in a process
  31. 31. Effect of the drug taxol on microtubule organization treatment of cancers
  32. 32. Actin and tubulin are highly conserved: they have to bind to many proteins directly and indirectly Accessory proteins and intermediate filament proteins are not as conserved
  33. 33. A model of intermediate filament construction Intermediate filaments are only found in some metazoans:vertebrates, nematodes,molluscs Not required in every cell type Ancesters: nuclear lamins Parallel Antiparrel No polarity! “subunit” 8 parallel protofilaments Easily bent Hard to break
  34. 34. Two types of intermdiate filaments in cells of the nervous system Neurofilaments:axons NF-L, NF-M, NF-H proteins coassemble axon glia NF-M and NF-H have long C-terminal tails That bind to neighboring filaments:uniform spacing Regular spacing When axons grow, subunits are added at the filament ends and along the filament length; axon diameter increase 5 fold In ALS (Lou Gehrig’s Disease), there is an accumulation and abnormal assembly of Neurofilaments in motor neuron cell bodies and axon--interfere with normal axon transport
  35. 35. Summary 1. Three types of cytoskeletal filaments, protofilaments; 2. Subunits, polymerization, treadmilling, dynamic instability; 3. Intermediate filaments, cell integrity, diseases caused by mutations in the intermediate filament genes 4. Natural toxins and cytoskeleton
  36. 36. Cilia and Flagella • Microtubules control the beating of cilia and flagella, locomotor appendages of some cells • Cilia and flagella differ in their beating patterns VViiddeeoo:: CChhllaammyyddoommoonnaass VViiddeeoo:: PPaarraammeecciiuumm CCiilliiaa Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  37. 37. Fig. 6-23 5 μm Direction of swimming (a) Motion of flagella Direction of organism’s movement Power stroke Recovery stroke (b) Motion of cilia 15 μm
  38. 38. • Cilia and flagella share a common ultrastructure: – A core of microtubules sheathed by the plasma membrane – A basal body that anchors the cilium or flagellum – A motor protein called dynein, which drives the bending movements of a cilium or flagellum Animation: CCiilliiaa aanndd FFllaaggeellllaa Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  39. 39. Fig. 6-24 0.1 μm Triplet (c) Cross section of basal body 0.5 μm Microtubules Plasma membrane Basal body (a) Longitudinal section of cilium (b) Cross section of cilium Plasma membrane Outer microtubule doublet Dynein proteins Central microtubule Radial spoke Protein cross-linking outer doublets 0.1 μm
  40. 40. • How dynein “walking” moves flagella and cilia: − Dynein arms alternately grab, move, and release the outer microtubules – Protein cross-links limit sliding – Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  41. 41. Intermediate Filaments • Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules • They support cell shape and fix organelles in place • Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  42. 42. Cell Walls of Plants • The cell wall is an extracellular structure that distinguishes plant cells from animal cells • Prokaryotes, fungi, and some protists also have cell walls • The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water • Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  43. 43. • Plant cell walls may have multiple layers: – Primary cell wall: relatively thin and flexible – Middle lamella: thin layer between primary walls of adjacent cells – Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall • Plasmodesmata are channels between adjacent plant cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  44. 44. Fig. 6-28 Secondary cell wall Primary cell wall Middle lamella Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata 1 μm
  45. 45. Cytoskeletal filaments are all constructed from smaller protein subunits Intermediate filaments: smaller elongated and fibrous subunits Actin and microtubule filaments: compact and globular subunits All form as helical assemblies of subunits Noncovalent interactions: rapid assembly and disassembly
  46. 46. Nucleation • Gamma tubulin in MTOC/centriole- MT’s grow from there
  47. 47. cenriole! Like MTOC/
  48. 48. MT’s are a highway-bringing things out and back from the center of the cell. /
  49. 49. Cilia action • Cilia= short, many • Flagella= long, few; NOT the same as the bacterial flagellum!!
  50. 50. 9+2; nexin, radial spokes, dynein A,B
  51. 51. The different sides of the cilium may slide, depending on the direction of sliding. These sliding more These sliding more
  52. 52. Intermediate filaments • 10 nm in diameter • Only in animals! (??plant/fungal nucleus??) • Variety of types- 60 genes! • Seem to be involved in providing strength to cells. • Able to interact with both MT's and microfilaments (actin filaments).
  53. 53. Intermediate filaments impart mechanical stability to animal cells Keratin filaments in epithelial cells “desmosomes” The most diverse family 20 in human epithelial cells 10 more in hair and nails Diagnosis of epithelial cancers (carcinomas)
  54. 54. Octamers of Tetramers make up the structure. No polarity! Subunits are filamentous, rather than globular.
  55. 55. • Keratin- epithelial cells, hair, nails • Neurofilaments- in, well, nerves • Lamins- lines the nucleus
  56. 56. When they are mutant • Smaller nerve fibers- a natural mutant quail! • Fragile skin • Sometimes muscle weakness • Sometimes nothing!
  57. 57. Microfilaments (Actin Filaments) • Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits • The structural role of microfilaments is to bear tension, resisting pulling forces within the cell • They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape • Bundles of microfilaments make up the core of microvilli of intestinal cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  58. 58. Fig. 6-26 Microvillus Plasma membrane Microfilaments (actin filaments) Intermediate filaments 0.25 μm
  59. 59. • Microfilaments that function in cellular motility contain the protein myosin in addition to actin • In muscle cells, thousands of actin filaments are arranged parallel to one another • Thicker filaments composed of myosin interdigitate with the thinner actin fibers Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  60. 60. • Localized contraction brought about by actin and myosin also drives amoeboid movement • Pseudopodia (cellular extensions) extend and contract through the reversible assembly and contraction of actin subunits into microfilaments Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  61. 61. • Cytoplasmic streaming is a circular flow of cytoplasm within cells • This streaming speeds distribution of materials within the cell • In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming Video: CCyyttooppllaassmmiicc SSttrreeaammiinngg Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
  62. 62. Microfilaments (Actin) • Where we’re going: • Basic structure, polarity, treadmilling • Muscle contraction • Amoeboid movement
  63. 63. Minus end Domains 1-4 ATP binding cleft Subunits= G actin-bound w/ATP; F-actin= microfilaments Looks like a double helix!
  64. 64. S1 is a myosin fragment that binds to actin- the points point to the minus end
  65. 65. Treadmilling of actin filaments actin sub units can flow through the filaments by attaching preferentially to the (+) end and dissociating preferentially from the (-) end of the filament. This treadmilling phenomenon occur in some moving cells.The oldest subunits In treadmilling filament lie at the (-) end.
  66. 66. Treadmilling-it’s easier to add to the + than – end at any concentration, and at some concentrations it’s adding at the + end at the rate it’s coming off the – end= treadmilling.
  67. 67. The treadmilling of an actin filament Structural difference between the two ends D form polymer leans towards disassembly
  68. 68. Muscle Contraction • Three types of muscle fibers: • Skeletal, striated, voluntary • Heart- more like skeletal, but not multinucleated. Its structure allows the propagation of an action potential (the heart beats by itself, w/o outside signals) • Involuntary, smooth muscle- gut, uterus, etc.
  69. 69. Multinucleated cell, arises from fusion; great big thing- 100mm X 100 um! 2.5 uM length Muscle contractility
  70. 70. These are myofibrils
  71. 71. Electron micrograph of sarcomere with bands lettered
  72. 72. The functional anatomy of muscle fibere
  73. 73. Myosin I, hauling a vesicle
  74. 74. Microtubules= interstate; actin= side roads
  75. 75. Light band http://www.youtube.com/wDaatcrhk ?bva=n0dkFmbrRJq4w Light band
  76. 76. Troponin binds Ca++, moves the tropomyosin 1.5 nm- myosin binds
  77. 77. Actin accessory proteins
  78. 78. ARP Filamin Fimbrin gelsolin Profilin (thymosins)

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