1 / 32

The Cytoskeleton

The Cytoskeleton. Functions Structural scaffold creating and supporting cell shape Framework positioning organelles within cytoplasm Network of molecular “ roads ” for intracellular transport of materials Framework for whole cell movement Framework for cell division. The Cytoskeleton.

sasson
Télécharger la présentation

The Cytoskeleton

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The Cytoskeleton • Functions • Structural scaffold creating and supporting cell shape • Framework positioning organelles within cytoplasm • Network of molecular “roads” for intracellular transport of materials • Framework for whole cell movement • Framework for cell division

  2. The Cytoskeleton • Three major structural components • Microtubules • Major role: support, intracellular transport • Intermediate filaments • Major role: mechanical strength to resist physical stresses • Microfilaments • Major role: muscle contraction, motility

  3. The Cytoskeleton • Microtubules (MTs) • Major role • Intracellular transport • Motor proteins drag cargo along them • Structural support • Resist compression forces • Resist shear (bending) forces • Hollow, rigid • 25nm diameter, 4nm wall thickness • Radiate outward toward plasma membrane from near nucleus (MTOC)

  4. The Cytoskeleton plus-end • Microtubules (MTs) • Unit = alpha / beta tubulin heterodimer • alpha subunit + beta subunit • Heterodimer is asymmetric • Beta end is called “plus” end • Alpha end is called “minus” end • Not referring to a charge difference minus-end

  5. The Cytoskeleton • Microtubules (MTs) • alpha / beta (a/b)-tubulin heterodimer • Beta subunit is a GTPase • Assembly • Polymer grows by addition of units at the “plus” end • GTP-bound tubulin can add • GTP form hydrolyzes to GDP form • GDP-bound tubulin cannot add • GDP-bound tubulin can release only from “plus” end • GDP-bound tubulin cannot release from “minus” end or from central region

  6. The Cytoskeleton • Dynamic instability • MTs can assemble/disassemble at different rates in different locations within a single cell • Various proteins can bind and stabilize MTs

  7. The Cytoskeleton • Microtubule-associated proteins (MAPs) • Form bridges crosslinking adjacent MTs for parallel alignment • Increase MT stability • Promote assembly • Regulated by phosphorylation state Anti-tubulin antibody stain

  8. The Cytoskeleton • Microtubule organizing centers (MTOCs) • GTP-bound a/b-tubulin spontaneously assembles into MTs very slowly • GTP-bound a/b-tubulin add to an existing MT very rapidly • MTOCs are the nucleation points for MT assembly • Centrosome • Basal body

  9. The Cytoskeleton • Microtubule organizing centers (MTOCs) • Centrosome • 2 centrioles at right angles to each other near nucleus • Contain gamma-tubulin subunit • Nucleate “minus” end of a/b-tubulin • Plus-end is oriented outward toward plasma membrane

  10. The Cytoskeleton • Microtubule organizing centers (MTOCs) • Basal body • Single centriole at the base of cilia and flagella

  11. Eukaryotic cilia and flagella • Hair-like motile organelle projecting from cell surface • Covered by plasma membrane

  12. Eukaryotic cilia and flagella • Central protein core is called an “axoneme”

  13. Eukaryotic cilia and flagella • Central protein core is called an “axoneme” • Composed of 11 MTs arranged in a “9+2” array • 9 outer MTs • 2 central MTs • Connected by various MAPs • Locomotion caused by sliding outer tubules past each other • Action of motor proteins (dynein)

  14. The Cytoskeleton • Motor proteins that “walk” on MTs • Kinesin gene family • Plus-end directed • Outward or “anterograde” transport • Dynein gene family • Minus-end directed • Inward or “retrograde” transport

  15. The Cytoskeleton • Kinesins are composed of 2 heavy and 2 light polypeptides • Cargo-interaction domain “tail” • Different kinesins have different specificities • ATPase “head” • Binds to MT • ATP hydrolysis propels heads forward • Highly processive

  16. The Cytoskeleton • Kinesins are composed of 2 heavy and 2 light polypeptides • ATPase “head” • Binds to MT • ATP hydrolysis propels heads forward • Highly processive

  17. The Cytoskeleton • Motor proteins that “walk” on MTs • Dynein gene family • Minus-end directed • Inward or “retrograde” transport • Very large (1.5MDa) • Involved in cilia/flagella movement

  18. The Cytoskeleton • Three major structural components • Intermediate filaments (~65 genes) • Major role: mechanical strength to resist physical stresses • Hemidesmosomes and desmosomes

  19. Intermediate filaments (IFs) • Animal specific • Strong, rope-like

  20. Intermediate filaments (IFs) • Animal specific • Strong, rope-like • Bridged together with other cytoskeletal elements • (e.g. plectin crosslinks MTs and IFs)

  21. The Cytoskeleton • Intermediate filaments • Composition and assembly • Monomers form dimers • Dimers form tetramers lacking polarity • Tetramers form larger fibers • Incorporation into existing filaments not limited to end regions

  22. The Cytoskeleton • Three major structural components • Microfilaments (MFs) • Major role: muscle contraction, motility • Solid, branched • 8nm diameter • Molecular unit= actin

  23. The Cytoskeleton • Microfilaments (MFs) • Actin molecule is asymmetric • “plus”-end versus “minus”-end • Actin is an ATPase • ATP-bound actin can be incorporated into growing MFs • plus-end of MFs grows 10x faster than minus-end • Higher dissociation rate from minus-end leads to treadmilling

  24. The Cytoskeleton • Microfilaments (MFs) • Drugs • Cytochalasin D blocks plus-end addition leading to complete MF depolymerization • Phalloidin blocks turn-over locking MFs into polymerized state + cytochalasin D

  25. The Cytoskeleton • Actin binding proteins + cytochalasin D

  26. The Cytoskeleton • Motors that walk on Microfilaments (MFs) • Myosin gene family • ATPase “head” domain • Cargo-interacting “tail” domain

  27. The Cytoskeleton • Motors that walk on Microfilaments (MFs) • Myosin gene family • Type V can walk on actin filaments carrying a bound cargo • Type II forms bipolar filaments via tail - tail interactions

  28. The Cytoskeleton • Myosin type II in muscle contraction • Muscle fiber • Large cell, 100mm long, 10-100 microns thick • Contain >100 nuclei • Derived from the fusion of many myoblast cells • Myofibrils • thin protein strands composed of repeating units called “sarcomeres” that give muscle its “striated” appearance • Sarcomere • Z, I, A, H and M regions

  29. Sliding filament model of muscle contraction

  30. Sliding filament model of muscle contraction

More Related