13 Actin - BME 418 Quantitative Cell Biology#13 Actin...

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BME 418, Quantitative Cell Biology Alan J. Hunt #13: Actin cytoskeleton S.E.M. of Bundles and networks of actin filaments in a cell after plasma membrane has been removed by detergent . I. INTRODUCTION A. Until a few decades ago, the cytosol of the eukaryotic cell was regarded as a relatively uninteresting, gel-like substance in which the organelles were suspended. B. Advances in microscopy and other techniques have revealed that the interior of eukaryotic cells is highly structured, containing a complex network of interconnected filaments, or cytoskeleton, extending throughout the cytosol. C. The cytoskeleton is apparently absent from bacteria and its evolution is a crucial development in the evolution of eukaryotic cells. The larger size and greater structural complexity of eukaryotes necessitates a support system to maintain structure and support morphologic changes. Additionally, the cytoskeleton serves as a set of rails so that cell constituents can be targeted and moved to appropriate locations. D. Problems with the cytoskeleton have been shown to cause disorders of the skin, the nervous system, and the muscles. Changes in the cytoskeleton are key, and even diagnostic, in the pathology of some diseases, including cancer. 1
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BME 418, Quantitative Cell Biology Alan J. Hunt E. The cytoskeleton provides an architectural framework conferring a high level of internal organization, enabling eukaryotic cells to assume and maintain complex shapes that would otherwise not be possible (eg. axons, extremely elongated processes of neurons). The term cytoskeleton does not convey its dynamic, changeable nature, being assembled and disassembled in a carefully regulated fashion and sometimes at astonishing rates. II. CYTOSKELETON A. Functions 1. cell shape 2. motility 3. cell division 4. positioning and movement of organelles and other cellular components 5. migration in the vertebrate embryo 6. polarity 7. intercellular contacts to produce tissue architecture (links to external environment, e.g. extracellular matrix). B. Elements of the Cytoskeleton 1. Microfilaments (also called F-actin or actin filaments) - polymers of G-actin monomers. F-actin is polymeric filaments made up of G-actin. - Filaments are about 8 nm in diameter, can be many microns long. - Relatively flexible, persistence length =17.7 microns (flexural rigidity = 7.3 x 10 -26 Nm 2 ) A less geometry-dependent way to express this is: T = B / r where T is torque, r is the radius of curvature, and B is flexural rigidity. 2. Microtubules - polymers of tubulin dimers (alpha and beta subunits). - Hollow tubes, about 25 nm in diameter, can be many microns long. - Rigid, persistence length = 5 mm (flexural rigidity = 2.2 x 10 -23 Nm 2 , comparable to if they were made of Plexiglas). How was this determined? 3. Intermediate filaments - A broad class of polymers formed from related proteins. Classified into 6 major types.
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