Chapter 9- the cytoskeleton

1. Chapter 9- the cytoskeleton

2. Where we’re going • Framing the problem- cells aren’t just bags! • Three types of cytoskeletal components • Cilia movement • Muscle movement • Amoeboid movement

5. 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.

7. 13 protofilaments

10. Nucleation • Gamma tubulin in MTOC/centriole- MT’s grow from there

13. Like MTOC/ cenriole!

14. MT’s are a highway- bringing things out and back from the center of the cell. http://www.mpasmb-hamburg.mpg.de/ktdock/

16. F. MICROTUBULE DYNAMICS: 9.25 • http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.box.2966 • 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!

17. W/ cap- slowly growing; w/o- rapidly shrinking- MT catastrophe!

18. Dynamic instability!

19. 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

20. MT drugs • Colchicine- prevents MT formation- arrests cells at metaphase • Taxol-Stabilizes MT’s – cancer drug • Useful in determining role of MT’s in a process

21. Cilia action • Cilia= short, many • Flagella= long, few; NOT the same as the bacterial flagellum!! (the bacterial flagellum is WAY cooler!)

24. 9+2; nexin, radial spokes, dynein A,B

25. Just in case you were wondering… dynein/kinesin also transport things up and down the cilium/flagellum

26. Slide-to-bend model for movement.

27. The different sides of the cilium may slide, depending on the direction of sliding. These sliding more These sliding more

28. Regulation • Beats 10-40X /sec! • They beat together! • The central pair may rotate, which send the signal (and simply moves the problem to how these rotate) • Weird paramecium surgery experiments • https://www.youtube.com/watch?v=vQ3CdSiVzUk

29. 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).

31. Octamers of Tetramers make up the structure. No polarity! Subunits are filamentous, rather than globular.

32. Some types to remember • Keratin- epithelial cells, hair, nails • Neurofilaments- in, well, nerves • Lamins- lines the nucleus

33. When they are mutant • Smaller nerve fibers- a natural mutant quail! • Fragile skin • Sometimes muscle weakness • Sometimes nothing!

35. Microfilaments (Actin) • Where we’re going: • Basic structure, polarity, treadmilling • It’s good buddy, myosin, w/ all its types • Muscle contraction • Amoeboid movement

36. Minus end ATP binding cleft Domains 1-4 Subunits= G actin-bound w/ATP; F-actin= microfilaments Looks like a double helix!

37. S1 is a myosin fragment that binds to actin- the points point to the minus end

38. 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.

39. Poisons! • CytochalasinB-depolymerizes • Phalloidin- stabilizes. The Amanita mushroom has TWO nasty toxins that cell biologists like. We’ll meet alpha amanitin later.

40. Heeelp! I’ve been poisoned! w/ cytochalasin! These are sea urchin cells. The projections are dissolving

41. Myosin- Actin’s good buddy • CONVENTIONAL: This is the muscle actin- aka Type II • UNCONVENTIONAL: These are the rest- types I & III- XV (named by the same people who named the Super Bowl) • Conventional: BIG molecule- ½ million MW! Looks like two golf clubs wrapped around each other. There are also two types of light chains (9.48) The tails are able to aggregate, producing a bipolar filament (9.50) • The unconventional myosins do things in ordinary cells; some are motors, hauling vesicles along actin filaments. If you believe 9.53, the MT’s act as main highways, and the actin as side roads at the ends of the cell.

44. Myosin I, hauling a vesicle

45. Microtubules= interstate; actin= side roads

46. 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.

47. Multinucleated cell, arises from fusion; great big thing- 100mm X 100 um! 2.5 uM length

48. These are myofibrils

50. Light band Dark band Light band http://www.youtube.com/watch?v=0kFmbrRJq4w