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Unformatted text preview: Cytoskeleton I (Actin filament) (Lodish Chapter 17) I. Functions of the cytoskeleton A. Cell motility. B. Movement of intracellular components 1. Vesicular trafficking 2. Chromosomes C. Regulation of cell shape II. Three major types of cytoskeletal fibers (Lodish Fig. 17-2) A. Microfilaments - Major component are actins, 5-9 nm in diameter. B. Microtubules – α- and β-tubulin, 25 nm in diameter. C. Intermediate filaments – several types of major proteins, ~10 nm in diameter. 1. Lamins – found in nucleus 2. Keratin – epithelial cells 3. Desmin – muscle cells 4. Vimentin – mesenchymal cells D. Lodish Fig. 17-2 and 17-4 show the distribution of cytoskeletal fibers in a typical eukaryotic cell. III. Actin microfilaments A. Highly conserved (375 aa) and present at high concentrations in the cell (0.1 – 0.5 mM). 1-5 % of all cellular protein (10% in muscle). B. Three types (though nearly identical in sequence) 1. α-actin – three isoforms found in different muscle cells 2. β-actin – found at the leading edge of motile cells 3. γ-actin – two isoforms, the major one found in stress fibers and the other found in intestinal muscle cells. C. Actin Assembly (Lodish Figs. 17-5 - 10) 1. Globular Monomers call G-actin (Lodish Fig. 17-5a). Bound to ATP. 2. Filamentous actin called F-actin (Lodish Fig. 17-5bc) 3. In the presence of Mg++, K+ or Na+ ions, G-actin assembles into F-actin. Process is reversible at low ionic concentrations. This is perhaps the most important properties of actin. 4. F-actin molecules hydrolyze ATP to ADP. D. F-actin filaments, actin bundles and actin networks 1. F-actin filaments have polarity – the end with the exposed ATP binding site is the minus end (Lodish Fig. 17-6). 2. Actin filaments often form bundles associated with the plasma membrane; while the actin network gives the cytosol gel-like properties. 3. Actin microfilament is thin, but actin cytoskeleton, the network of the actin filaments, is huge. It controls cell shape and movement. IV. Dynamics of actin assembly (Lodish Fig. 17-7 - 17-16) A. Three phases (Lodish Fig. 17-7) 1. nucleation – nuclei are stable actin polymers (usually 4 subunits long) 1 2. elongation – filaments rapidly increase in length 3. steady state – filament elongation and dissociation reach equilibrium B. Concentration of actin determines whether filaments form or not 1. Critical concentration (Cc) is point above which filament formation can occur (Fig....
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- Winter '08
- cell biology, Lodish Fig., Lodish Figs.