L12 - Exams will be distributed at the end of class We are...

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Exams will be distributed at the end of class We are on chapter 16: The cytoskeleton Pages: 965-988, 992-1009, 1014-1022, 1031-1034, 1036-1043. Today we will Fnish actin binding proteins and then move onward to look at functional importance (cell movement). ±inal small mention of intermediate Flaments (brie²y).
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A clarifcation: Eucaryotic Fagella (protozoans to humans) have Fagella that wave in undulating motion that we went through in last class. Bacteria have Fagella that do not contain microtubules or dynein and do not wave. These Fagella rotate like propellers, driven by a special rotary motor embedded in the bacterial wall.
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There are three primary components of the cytoskeleton: There are three major networks of filamentous fibers inside the cell 2- microfilaments - made of actin subunits - 5 nM across Responsible for cell shape and movement Kalil lab U. Wisconsin 2-stranded helical polymers
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ACTIN ASSEMBLY Actin monomers come together to make microfilaments: thin helical polymers of actin subunits --no hollow lumen! Microfilaments have polarity: plus end and a minus end. The process of assembling microfilaments involves ATP. Monomers of actin bind to ATP and then they can bind to microfilament Monomers bind to (+) end at ten times the rate that they bind to the (-) end of the microfilament Soon after actin binds to microfilament, ATP is hydrolyzed to ADP which remains bound to actin. After filaments are disassembled , actin must exchange ADP for ATP in order to form filaments again. This process of switching ADP for ATP takes on the order of minutes, which explains why about 50% of the actin in the cell exists as monomers.
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Similarities in actin and microtubule assembly Actin and tubulin subunits hydrolyze nucleoside triphosphates after they assemble into polymer. The role of ATP hydrolysis in actin polymerization is analogous to the role of GTP hydolysis in microtubule polymerization. Free actin subunits (i.e. subunits that have not assembled into polymer) bind ATP, and hydrolyze the ATP to ADP after polymerization. Free tubulin subunits bind GTP, and hydrolyze GTP to GDP after polymerization. Hydrolysis of the bound ATP or GTP causes conformational changes in the subunits. Subunits within the polymer can therefore exist in two forms: the T form, or the D form. Subunits in the T form bind more tightly to each other and favor polymerization. Subunits in the D form bind less tightly to each other and favor depolymerization. The bound ATP or GTP are hydrolyzed some time after the subunits have been incorporated into the polymer. The longer a subunit has been in the polymer, the more likely it will have hydrolyzed its bound nucleoside triphosphate. ATP actin usually found at ends of actin. ATP is required for assembly. Like tubulin,
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L12 - Exams will be distributed at the end of class We are...

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