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Published byΔιόδωρος Βλαχόπουλος Modified over 6 years ago
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CYTOSKELETON intermediate filaments: nm diameter fibers - rope-like assemblies of IF proteins - provide mechanical strength microtubules (axonemal, cytoplasmic): - 25 nm diameter cylinders - polymers of a, b-tubulin - organize organelles, direct intracellular transport actin filaments (microfilaments): - 5-9 nm diameter filaments (F-actin) - polymers of actin (G-actin) - concentrated in cell cortex - determine shape of cell surface, whole cell motility accessory proteins methods
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Intermediate filaments are staggered arrays of coiled coil proteins
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Distinct IFs in different cell types (Table 16-1)
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Tubulin: monomers bind GTP; GTP-GDP in polymer (b monomer only)
13 protofilaments
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Actin: monomer binds ATP; ATP-ADP in polymer
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Polymerization in vitro:
- critical concentration - time course of polymerization - plus, minus ends (Panel 16-2)
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- MTs growing from a stable MT
bundle (core of a cilium): plus end grows faster
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Hydrolysis of GTP or ATP in polymer:
- results in treadmilling (actin filaments) or dynamic instability (MTs)
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Treadmilling of a microtubule in vivo
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Dynamic instability of microtubules:
catastrophe and rescue
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Dynamic instability of MTs:
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Dynamic instability of MTs in vivo
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Drugs that affect actin and MTs (Table 16-2)
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MT polymerization is nucleated by
g-tubulin ring complexes (g -TuRC)
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MTOCs: - animal cells: centrosome with centrioles - plants, fungi: no centrioles, MTOCs in nuclear envelope
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EM of centrosome
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Actin filaments assemble at the cortex (nucleated at the plasma membrane):
a) all actin filaments - most formed before cells were permeabilized (fluor-phalloidin) b) newly formed filaments - 5 min. after labeling (rhod-actin monomers)
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ARP 2/3 complex: nucleates actin filaments from the minus end
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Actin-binding proteins organize actin filaments in cells:
- nucleate - bind G-actin (sequester or promote monomer addition) - bind filament sides (stabilize or destabilize) - cap (+ or - ends) - sever - bundle (loose or tight; parallel or anti-parallel) - crosslink - branch - link to membranes (direct or via membrane proteins - regulate MT-binding proteins
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Actin binding proteins in yeast
(genetics or biochemistry)
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- bind G-actin
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Bind sides of actin filament:
- tropomyosin (stabilizes) - cofilin (destabilizes ADP-bound filaments)
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+ end capping (CapZ): - filament grows more slowly above critical conc. - filament shrinks more slowly below critical conc.
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Actin cross-linking proteins - conserved actin-binding sites:
- bundle (a-actinin, fimbrin) - cross-link at right angles (filamin) - web-forming (spectrin)
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Actin-severing: - sever only: accelerates assembly of new filaments - sever and cap + end (gelsolin): slows filament growth
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(extracellular matrix receptor)
Attach actin filaments to plasma membrane: - mediate cell adhesion
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- microinjection of constitutively active forms
Actin cytoskeleton: global responses to extracellular signals depend on Rho family proteins (Ras superfamily - small monomeric G-proteins)
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