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Unformatted text preview: 1025-1050 the cytoskeleton and cell behaviorThe cytoskeleton and cell behavioroSliding of Myosin II and Actin Filaments Causes Muscles to ContractMyofibril: a cylindrical structure that is often as long as the giant muscle cell itself. It consists of a long repeated chain of tiny contractile units called sacromeresDark bands contain myosin (thick), light bands contain actin (thin)Plus ends of actin are attached to z discsLight bands have lower density of proteinZ disc caps the filaments and prevents depolymerizationZ disc holds filaments togetherNebulin stretches from Z disc to minus end of thin filament, acts as a molecular rulerMinus ends of thin filaments are capped and stabilized by tropodoulinTitin acts as a molecular springoA Sudden Rise in Cytosolic Ca2+ Concentration Initiates Muscle ContractionThick/thin interaction only happens when signal goes from skeletal muscle from its motor nerve.2 features for muscle cell rapid contraction:Individual myosin motor. Filament sliding driven by the ATPase of the myosin motor domainCa2+ pumping driven by the Ca2+ pump Incoming AP activates Ca2+ channel in T-tubule membrane, a Ca2+ influx triggers the opening of Ca2+ release channelsTroponin complex of 3 polypeptides (troponin T, I and C)In resting muscle, the troponin I-T complex pulls the tropomyosin out of its normal binding groove into a position along the actin filament that interferes with the binding of myosin headsWhen level of Ca2+ is raised, troponin C causes troponin I to release its hold on actin, which allows the tropomyosin molecules to slip back into their normal position so that the myosin heads can walk along the actin filaments-----------------------------------------------------------------------------------Smooth muscle cells dont express troponinsInstead, Ca2+ influx into the cell regulates contraction by 2 mechanisms that depend on the ubiquitous calcium binding protein calmodulinFirst, Ca2+ bound calmodulin binds to an actin binding protein, CALDESMON, which blocks the actin sites where the myosin motor heads would normally bindThis causes caldesmon to fall off of the actin filaments, preparing the filaments for contractionSecond, smooth muscle myosin is phosphorylated on one of its 2 light chains by myosin light chain kinase MLCKWhen the light chain is phosphorylated, the myosin head can interact with actin filaments and cause contractionWhen it is dephosphorylated, the myosin head tends to dissociate from actin and becomes inactiveMLCK requires bound Ca2+/calmodulin to be fully activeExternal signaling molecules like adrenaline also regulate the contractile activity of smooth muscleAdrenaline binds to its G protein coupled cell surface receptor which causes an increase in the intracellular level of cyclic AMP, which activates cyclic-AMP dependent protein kinase (PKA)PKA phosphorylates and inactivates MLCK, causing the smooth muscle cell to...
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This note was uploaded on 12/30/2010 for the course BIO 320 taught by Professor Staff during the Spring '08 term at University of Texas at Austin.
- Spring '08