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Topic 3D actin filaments.odt

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University of Ottawa

Topic 3DActin Filaments microfilamentsapprox 7 nmorganized into higher order structures forming bundles or 3D networks with properties of semisolid gelsparticularly abundant beneath the plasma membrane where they form a network to provide mechanical support determine cell shape allow movement of cell surfaceAssembly and Disassembly of Actin Filamentsextremely abundant in all types of eukaryotic cellsMammals have at least 6 actin genes 4 in different types of muscle 2 in nonmuscle cells Individual actin molecules are globular proteins Monomer is G actin which polymerizes to form filaments F actinhave distinct polarity plus and minus end in low ionic strength actin filaments depolymerize to monomers and polymerize spontaneously if ionic strength is increasedFirst step in polymerization is called nucleation and is the formation of an aggregate of 3 actin monomers Filaments then grow on both ends but the plus end elongates 510 times faster than the minus endActin monomers bind ATP which is hydrolyzed to ADPActin monomers bound to ATP polymerize more readily than those to which ADP is bound The rate at which monomers are incorporated into filaments is proportional to their concentration so there is a critical concentration at which the rate of polymerization equals the rate of dissociation treadmillingCytochalasins bind to plus end of actin filaments and block their elongation stopping cell divisionPhalloidin binds to actin filaments preventing their dissociationAssembly and disassembly is regulated by actinbinding proteinsKey protein cofilin which enhances the rate of dissociation from minus end and severs filament It binds to ADP actin keeping them in ADP bound formProfilin can reverse this by stimulating the incorporation of monomers into filaments Profilim stimulates the exchange of bound ADP for ATP resulting in the formation of ATPactin monomersOther proteins serve as nucleation sites to initiate the assembly of new filaments
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