Cytoskeleton

Cytoskeleton

1. Microtubules

2. Microfilaments

   • + / - can both be capped
     • CapZ = + cap
     • Tropomodulin = - cap
   • Skeletal Muscle
     • Sacromere = Z band , MF, M2 btwn them, I band, A band
     • Titin : binds M2 btwn the MFs
     • Nebulin : binds MF = stable
     • Ca+ = change chape of troponin --> change shape of tropomyosin =
       • exposed M2 binding domain
         • muscle contraction (as long as Ca+ present)
   • Smooth muscle
     • Involuntary
     • No troponin
     • Tropomyosin and calmodulin
     • Ca+ influx
       • Binds and activates calmodulin
         • Calmodulin Phosphorylates myosin light chains (LC)
           • P-LC are active = detangle heavy chains
             • Formation of myosin thick filament
               • Sacromere like contractile stucture
   • Made of actin monomers (G protein = 4 lobes + ATP cleft)
     • Polymerization needs : ATP G protein and [monomer] > Cc
       • - end Cc = 0.6 mM
       • + end Cc = 0.12 mM
       • Thymosin : sequesters G actin = prevents poly
       • Profilin : Promotes ATP G actin = promotes poly
       • Cofilin : enhances depolymerization
   • Drugs
     • Cytochalarin = depol
     • Phalloidin = stabilizes
   • Nucleation
     • Formin = Linear (unbranched)
       • Activated by Rho GTP
       • Tethered to plasma membrane
       • If fixed = "- end" grows
       • If moves : + end grows
     • Arp 2 / 3 = branched
       • Activated by Wasp which is activated by Cdc 42
       • Activated by WAVe which is activated by Rac
       • If fixed = participates in endocytosis
       • If not fixed = participates in phagocytosis
       • Tethered to the plasma membrane
   • Networks
     • Microvili + filipodia
       • a-actinin
       • Fimbrin
       • Anchored by ezrin
     • Circular networks
       • Ankryrin binds to spectrin which bind to MF
     • Filamin = stress fibers
     • Muscle contraction = myosin + actin
       • Pull on Dystrophin
         • Pull on dystroglycan complex = signal out
   • Motor protein = Myosin
     • Myosin 2 (muscle)
       • - Light chain = head (atp hydro), bind MF + neck (controlled by light), + heavy = tail
         • Overlap = myosin thick filaments
     • Myosin 5 (transport)
       • Same as 2 but has cargo binding tail
       • can bind to actin railways and move the cytoplasm = efficient diffusion
     • Myosin 1 : head only only move with atp
   • Cell migration
     • 1) chemotaxic signal = formation of a new lamelipodia or fillipodia
       • 2) New focal adhesion and stress fiber with myosin
         • 3) Stress fiber, myosin and MF translocate the cell body
           • 4) Deadhesion at the back edge
     • 1) Chemotaxic receptor receives signal
       • 2) Activates CDC42 which activates :
         • 2.1) PAR6 = polarity / direction (MT)
         • Rac which activates
           • Rho GTP
             • Formation + Myosin
               • Contractile stress fiber
           • Wave
             • Arp 2/3
               • Lamellipodia
         • 2.2) WAsp
           • Arp 2/3
             • Fillipodia formation

3. Intermediate Filaments

   • No polarity / slow dynamics / Tetramer subunit / no motor protein
   • Epithelial cells : keratin
   • Migratory cells : vimentin
     • Linked to membrane by ankyrin
   • Muscle cells : Desmin
   • Neurons : neurofilaments
   • Neuclei : Lamins (ABC)
     • Lamin B : linked through prenylation
     • Lamin A : desactivated by ser P
   • Provide structural strength
     • Cell-Cell Adhesion
       • Desmosomes
     • Cell-ECM Adhesion
       • Hemidesmosomes
   • Contain proteins linking them to IF/ MT (Plectin)
   • LINC Complexes : link IF, MT, MF to nuclear mebrane
   • Need to be cleared up before mitosis
     • Phos by mitotic cyclin

4. Adhesion

   • Cell-cell
     • Adherins junctions - Actin
     • Desmosomes - IF
   • Cell-ECM
     • Focal Adhesions - Actin
     • Hemidesmosomes - IF
   • Functional - Non structural
     • Gap Junctions
       • 2 connexons (2x6 connexins)
       • Open channel between cells
       • Closes when Ca+ passes through
     • Tight Junctions
       • Made of JAM, occludin, claudin
       • Near the apical surface
       • Glue cells together and avoid things passing in between
   • Interactions
     • Homophilic
       • Cell-cell
       • Cadherins
         • Ca+ Dependent
       • Ig superfamily
         • Ca+ independent
     • Heterophilic
       • Integrins
         • a1b1 = collagen
         • a5b1 = fibronectin
         • a6b1 = laminin
       • Selectins = oligosacharrides
         • Extravasation
           • 1) inflamation signal = P selectin out + paf prod
             • 2) P selectin binds oligo, slows leuko, Paf receptor bindin
               • 3) Activate integrin = binds icam
   • ECM
     • Hydrophobic
       • Peptidoglycan - absorb water
     • Structural
       • Collagen - tissue resiliency
     • Adhesive
       • Integrin, laminin - cell migration