Note Feb 4, 2013 recitation and lecture

Subunits that were incorporated into the polymer

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affinity for other subunits. Subunits that were incorporated into the polymer awhile ago are thus more likely to undergo disassembly than subunits that were just added. The structural polarity of the subunits causes the two ends of the filament to have different C c s. The steady state behavior of the filament is achieved at a point midway between the C c . for the two ends. At steady state, actin filaments undergo treadmilling , where ATP bound subunits ("T") are added at the plus end and ADP bound subunits ("D") are lost from the minus end. The actin concentration in most cells is much higher than the C c . The cell uses a large number of actin-binding proteins to help control polymerization and to organize actin filaments into higher order structures such as crosslinked gels and parallel bundles. Microtubule dynamics: Microtubules are hollow tubes whose wall is made up of a parallel array of 13 protofilaments of α / β dimers. Each tubulin dimer contains two bound GTPs (one GTP per monomer) but only the β subunit can exchange or hydrolyze its GTP. The plus end of the polymer terminates in β - tubulins. GTP hydrolysis causes a large conformational change in the tubulin dimer so it goes from relatively straight to curved. Curved dimers are not easily accommodated in linear The system will revamp itself to keep the concentration the same after a sample is diluted
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Cell Biology (020.306) CYTOSKELETON Weeks 2 and 3 3 protofilaments, so microtubules containing GDP-bound β -tubulin are unstable and tend to fall apart. Microtubule binding proteins help govern disassembly and can also crosslink MTs. The steady state condition for microtubules is called dynamic instability . Microtubules do not usually treadmill but instead exist in one of two states, growing or shrinking. During growth, addition of new tubulin dimers to the plus end creates a "GTP cap" which stabilizes the tip. If all the β -tubulin subunits in the cap hydrolyze GTP to GDP, the protofilaments are completely curved so they spring apart and disassemble rapidly. Conversion between growth and shrinkage is called catastrophe ; conversion between shrinkage and growth is called rescue . Dynamic instability gives the cell the flexibility to quickly reorganize its microtubule cytoskeleton in response to external stimuli or for assembly of different microtubule arrays such as the mitotic spindle. INTERMEDIATE FILAMENTS (IFs) IFs are not polar and are considerably less dynamic than F-actin or microtubules. Different tissues contain different intermediate filament proteins. Neurons contain neurofilaments (Type IV), epithelial tissues contain cytokeratins (Type I and Type II) and cells such as fibroblasts and muscle cells (mesenchymal derivatives) contain Type III IF proteins such as vimentin and desmin. All cells also contain nuclear lamins (Type V).
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