BIOL 4610 1st Edition Lecture 19Outline of Current Lecture 1. Cell Organization and Movement 2. Microfilaments and Actin Structures3. Dynamics of Actin Filaments4.Current LectureChapter 17 – Cell Organization and Movement I: MicrofilamentsCytoskeleton location and function-Network of proteinsà filaments 1. Holds the shape of the cell2. Attaches to plasma membrane and aids in some movements3. Holds organelles in place4. Tracks- for movement5. Contraction (skeletal muscle)Cytoskeleton is dynamic – We can change number of filaments, how big they are, and shape.Through signal transduction pathwaysCytoskeleton has 3 major filament systems –1) microfilaments – polymers of protein actin; used for overall shape of cell; microvilli; tracks for myosins –muscle contractionThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.2) microtubules– polymers of tubulin; used for cilia; flagella, mitosis, tracks for kinesins and dynamins; moves vesicles3) intermediate filaments – polymers of several monomers; tissue-specific manner (keratine, skin)17.1. Microfilaments and Actin StructuresStructures and locations: shape of cellMicrovilli- increase surface area for absorption, common in cell cortex- (immediately underliningcell membrane is actin) supports and shapes the plasma membraneIn epithelial cells you have adherens belt: maintains cell polarity. Belt is the delineation betweenthe apical and basolateral side. Migration- cell movement1. Filipodia- finger-like projections that stick out2. Lamellipodium- leading edge (like a hand), starts the cell movement3. Stress fibers; bring the rear of the cell along (like squeezing toothpaste)Actin is the basic building block; microfilament is actin (and associated proteins) in polymerized form. Actin is the monomer.G-actin versus F-actinG = globular, monomer formF = filamentus, polymer form 17.2. Dynamics of actin filamentsPolymerization of pure G-actin occurs in 3 steps:1. Nucleation: have 3-4 actin monomers attaching to one another1. Rate limiting step **2. Elongation: rapidly add monomers to both ends3. Steady state: overall length doesn’t change but still get addition/subtraction of monomersActin filaments grow faster on the (+) endPositive end favored for addition of new monomersBy 10XC- and C+ are the critical concentrations= above this concentration get addition/growth of filament. If go below the number, get shrinkage/loss of monomersTreadmilling: 10X addition favored on positive endGaining of monomers on one end and loss of monomers on other end (negative end)Ability to treadmill is aided by ATP- G actin- addition monomer à ATP actin, once actin is boundin a filament it rapidly hydrolyzes ATPProteins that aid in polymerization and treadmilling:1) Profilin- binds to G-actin on opposite side of the nucleotide binding clef- Acts to release the ADP;ATP binds insteadProvides a supply of ATP-bound Actin1) Cofilin- binds to actin filaments, but Only in areas where associated with ADP(negative end)à Induces a twist that destabilizes it so chunksof actin filament break of Proteins that aid in polymerization:3) Thymosin-b4: critical concentrtions: 0.12 – 0.6MIf above these levels, the monomers should polymerizeNormal concentrations of actin in cells is 100-200 Mà Wraps around actin monomer and prevents ability to bind to a filament. à This is how we can have a lot of actin ready to go but hold it backCapping proteins: bind to F-actin and stabilize it 1) CapZ- positive end; km: 0.1nMBinds to positive ends with very high affinity2) Tropomodulin- negative end; ** mainly found in skeletal muscle- places where we need to stabilize length of filamentBoth proteins are regulated by
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