Note Feb 4, 2013 recitation and lecture

The cytoskeleton provides rigidity to the cell it is

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movement and division. The cytoskeleton provides rigidity to the cell. It is able to do this even though it is flexible and able to assemble and disassemble rapidly. Three types of cytoskeletal filaments: • Microfilaments (actin monomers = 42kDa, bind ATP) • Microtubules (tubulin heterodimers; α -tubulin and β - tubulin, each 55kDa, bind GTP) • Intermediate filaments (different IFs, each made of a distinct protein subunit type) MICROFILAMENTS = F-ACTIN : G-actin monomers assemble into a single 6 nm diameter chain (no protofilaments) MICROTUBULES : Tubulin dimers assemble end-to-end into a linear protofilament. Thirteen protofilaments are aligned side-to-side to form a hollow tube, 25nm diameter INTERMEDIATE FILAMENTS : Four subunits form a tetramer, eight tetramers twist together, 10nm diameter Basics of filament structure: Subunits are associated into protofilaments and multiple protofilaments form filaments via side-to-side interactions. The associations are weak, non-covalent interactions. The resulting filaments are strong because each subunit contacts more than one other subunit (i.e., strength in numbers). Because the individual interactions are weak the resulting filaments can be induced to disassemble rapidly, which allows them to be dynamic. This allows the cell to respond to its environment. Cytoskeletal polymer subunit assembly into filaments can measured as changes in the turbidity or viscosity of the solution, or by centrifugation to separate polymer from the free subunits. The kinetics of polymer assembly show three distinct phases: (1) a nucleation phase , where a small number of free subunits come together to form a nucleus , (2) an elongation phase where subunits are adding rapidly to both ends of the filament and (3) steady state , where the overall Nucleation requirement: it uses special proteins to catalyze filament nucleation at specific sites, thereby determining the location at which new cytoskeleton filaments are assembled. Filament nucleation is a primary way for cells to control their shape and their movement
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Cell Biology (020.306) CYTOSKELETON Weeks 2 and 3 2 polymer mass remains constant. This is NOT a chemical equilibrium. At steady state, the polymer is assembling and disassembling so free subunits are in exchange with polymer. C ritical concentration (C c ) is the threshold concentration of free subunit required for filaments to form. Below the C c , filaments will not assemble. If filaments are diluted to a concentration below the C c they will disassemble. Microfilaments (F-actin) and microtubules are polar filaments: the two ends are biochemically and structurally different. The "plus" end assembles more rapidly than “minus” end. The inherent asymmetry of the protein subunit and the fact that the subunits hydrolyze the bound nucleotide both contribute to polarity. Only subunits that have assembled into filaments hydrolyze nucleotide triphosphates (ATP or GTP) to nucleotide diphosphates (ADP or GDP); free subunits do not. Nucleotide hydrolysis triggers a conformational change that lowers its affinity for other subunits.
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