Molecular Cell Biology Answer Set Chapters 17, 18, 19

Molecular Cell Biology Answer Set Chapters 17, 18, 19 - 17...

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56 17 Cell Organization and Movement I: Microfilaments Review the Concepts 1. Actin filaments (microfilaments) are composed of monomeric actin protein subunits assembled into a twisted, two-stranded polymer. Actin filaments provide structural support, particularly to the plasma membrane, and are important for certain types of cell motility. Microtubules are composed of α - and β -tubulin heterodimers assembled into a hollow, tubelike cylinder. Microtubules provide structural support, are involved in certain types of cell motility, and help generate cell polarity. Intermediate filaments are formed from a family of related proteins such as keratin or lamin. The subunits assemble to create a strong, ropelike polymer that, depending on the specific protein, may provide support for the nuclear membrane or for cell adhesion. 2. For actin filaments, polarity refers to the fact that one end is different from the other end. This difference is generated because all subunits in an actin filament point toward the same end of a filament. By convention, the end at which the ATP-binding cleft of the terminal actin subunit is exposed is designated the ( ) end, while the opposite end, at which the cleft contacts the next internal subunit, is termed the (+) end. Polarity may be detected by electron microscopy in “decoration” experiments in which myosin S1 fragments (essentially myosin head domains) are incubated with actin filaments. The S1 fragments bind along the actin filament with a slight tilt, leaving the actin filament decorated with arrowheads that all point toward one end of the filament. The pointed end corresponds to the ( ) end and the barbed end corresponds to the (+) end. 3. Cells utilize various actin cross-linking proteins to assemble actin filaments into organized bundles or networks. Whether the actin filaments form a bundle or a network depends on the specific actin cross-linking proteins involved and the structure of the actin cross- linking protein. Actin cross-linking proteins that generate bundles typically contain a pair of tandem (i.e., closely spaced) actin-binding domains, while actin cross-linking proteins that generate networks typically contain actin- binding sites that are spaced far apart at the ends of flexible arms. 4. Once actin has been purified, its ability to assemble into filaments can be monitored by viscometry, sedimentation, fluorescence spectroscopy, or fluorescence microscopy. The viscometry method measures the viscosity of an actin solution, which is low for unassembled actin subunits but increases as actin filaments form, grow longer, and become tangled. The sedimentation assay utilizes ultracentrifugation to pellet (sediment) actin filaments but not actin subunits, and thereby separates assembled actin from unassembled actin. Fluorescence spectroscopy measures a change in the fluorescence spectrum of fluorescent-tagged actin subunits as they assemble into actin filaments. Lastly, fluorescence microscopy can
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This note was uploaded on 04/01/2008 for the course MCDB 428 taught by Professor Wang during the Winter '08 term at University of Michigan.

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Molecular Cell Biology Answer Set Chapters 17, 18, 19 - 17...

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