Note Feb 4, 2013 recitation and lecture

The assembly unit of an intermediate filament is an

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The assembly unit of an intermediate filament is an anti-parallel tetramer composed of a pair of dimers. Dimer assembly (or heterodimers for keratins) involves formation of an α -helical coiled-coil. Tetramers pack end to end to form protofilaments. Eight protofilaments wrap If a solution concentration changes but the free flowing monomer concentration stays the same, the mass decreades
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Cell Biology (020.306) CYTOSKELETON Weeks 2 and 3 4 together to form the ropelike IF polymer. Coiled-coil interactions are driven by hydrophobicity. Drugs that affect filament stability/dynamics Natural compounds are available that plants and animals use as toxins or poisons, doctors use as drugs for patients, and scientists use for experiments. The drugs can favor the depolymerized state by binding to monomers, or they can bind to filaments to stabilize them. Regulating Filaments in Cells: Nucleating, Stabilizing, Organizing Most microtubule minus ends are capped by a protein complex called the γ -tubulin ring complex ( γ -TuRC). γ -tubulin is similar to α - and β -tubulin, but it does not polymerize into filaments. Microtubules are often organized in radial arrays emanating from a microtubule organizing center (MTOC; aka centrosome). The core structural component of the centrosome is an unusual tubulin assembly (9 triplet microtubules) known as the centriole . This is surrounded by pericentriolar material, which contains γ -TuRCs and other proteins. Some microtubule binding proteins complement the inherent properties of these filaments. Brain MAPs (microtubule-associated proteins) stabilize microtubules and may crosslink them at a fixed distance from one another. Stathmin sequesters subunits to help maintain a disassembled dimer pool to favor disassembly (like nocodazole). Katanin severs filaments. Plus-end binding proteins attenuate dynamics at the plus end by controlling catastrophe and rescue frequency. The binding proteins that interact with F-actin and microtubules play similar but fundamentally different roles. The high concentration of actin in cells allows actin to assemble into filaments readily. The cell uses actin-binding proteins like profilin to promote nucleotide exchange and filament assembly. Actin filaments are floppy and
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Cell Biology (020.306) CYTOSKELETON Weeks 2 and 3 5 randomly disorganized, so other actin-binding proteins like filamin are used to stiffen filaments and allow them to form gels and other arrays. Capping protein stabilizes F-actin +-ends. Formins and tandem monomer sequestering proteins associate with filament plus ends to facilitate assembly. The Arp2/3 complex organizes actin filaments into branched networks. Arps 2 and 3 are related to actin but do not form filaments by themselves. As components of the Arp2/3 complex, they bind along F-actin filaments and nucleate growth of new filaments by acting as a minus-end cap. All these can be activated in response to signals to trigger massive actin assembly at specific sites in cells, leading to large scale shape changes and cellular movement.
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