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Chapter 16 (Part 1) the Cytoskeleton

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Chapter 16 (Part 1) The Cytoskeleton CONTENT Function and origin of the cytoskeleton Actin and actin-binding proteins Myosin and actin Lecturer: Yu-Ling Shih, IBC, AS FUNCTION AND ORIGIN OF THE CYTOSKELETON • Types of cytoskeleton (best known functions) Microfilament (cell shape, locomotion, cytokinesis) Microtubule (organelle position, intracellular transport, chromosome segregation) Intermediate filament (mechanical strength) • Organization of cytoskeleton Polarity, dynamic 3D networks, constantly remodel (assemble/disassemble) • For each type of cytoskeleton, you need to know- Subunit, nucleotide usage, dynamic and mechanical properties, cellular localization, motor and associating proteins, functions Introduction Cultured cell Dividing cell Green- microtubule Green- microtubule Red- microfilament Red- Intermediate filament Blue- DNA Blue- DNA Three MAjor Types oF Protein FilAments thAt Form the Cytoskeleton Three MAjor Types oF Protein FilAments thAt Form the Cytoskeleton Three MAjor Types oF Protein FilAments thAt Form the Cytoskeleton FUNCTION AND ORIGIN OF THE CYTOSKELETON • Summary of microfilament-based structures: microvilli cell cortex stress fiber focal adhesion lamellipodium (leading edge) adhesion belt contractile ring Diagram of changes in cytoskeletal organization associated with cell division • The polarization of the microtubule cytoskeleton is assisted by the microtubule- organizing center (MTOC) located in front of the nucleus. • When the cell divides, the polarized microtubule Lamellipodia Filopodia array rearranges to form a bipolar mitotic spindle, which is responsible for aligning and then segregating the duplicated chromosomes. • The actin microfilaments form a contractile ring Contractile ring at the center of the cell that pinches the cell in two after the chromosome segregation. Green- microtubule Red- microfilament Brown- DNA DynAmic And constAntly remodeled Organization of the cytoskeleton in polarized epithelial cells Microfilament: microvilli, cell adherens junction IF: link to adhesive structure, ECM Microtubule: Vesicle/organelle transport The bacterial cytoskeleton • Tubulin homolog FtsZ- Cytokinesis CreS • Actin homolog MreB- Cell morphology • Actin homolog ParM- PlAsmid segregAtion • IntermediAte filAment Crescentin- Cell morphology FtsZ MreB ParM ACTIN STRUCTURE AND POLYMERIZATION DYNAMICS • Actin monomer, globular actin- G-actin • Filamentous polymer – F-actin • Filament and functional polarity (-) end: pointed end (+) end: barbed end • Actin subunits assemble head-to-tail to create flexible, polar filaments • Polymerization dynamics nucleation-> elongation -> steady state -> treadmilling • Nucleation is the rate-limiting step in the formation of actin filaments. • Actin filaments have two distinct ends that grow at different rates. • ATP hydrolysis within actin filaments leads to treadmilling at steady state. The structures of an actin monomer and actin filament MoleculAr Asymmetry Structural polarity of the actin filament • Myosin S1 domain The Polymerization of Actin and Tubulin- ON Rates and OFF Rates Nucleation Is the Rate-Limiting Step in the Formation of Actin Filaments The Polymerization of Actin and Tubulin- The Critical Concentration The Polymerization of Actin and Tubulin- The Time Course of Polymerization Nucleation Is the Rate-Limiting Step in the Formation of Actin Filaments Gibbs free energy Protein conformational changes induced by - • Nucleotide binding • Hydrolysis • Phosphate release The Polymerization of Actin and Tubulin- ATP Caps and GTP Caps Dynamic Instability and Treadmilling The Polymerization of Actin and Tubulin- Treadmilling The time course of actin polymerization in a test tube ATP Hydrolysis Within Actin Filaments Leads to Treadmilling at Steady State The Polymerization of Actin and Tubulin- Dynamic Instability treAdmilling ACTIN AND ACTIN-BINDING PROTEINS • Actin-binding proteins inFluence Filament dynAmics And orgAnizAtion • Monomer AvAilAbility controls Actin FilAment Assembly • Actin-nucleating Factors accelerate polymerization and generate branched or strAight FilAments • Function Monomer-binding: Profillin, Cofillin, Thymosin β4 Nucleation and elongation: Formin Branching nucleation: Arp2/3 Reaction of filament- Capping: CapZ Severing: Cofillin, Gelsolin Crosslinking: Fimbrin, α-actinin, Spectrin, Filamin Stabilization: Tropomyosin Actin-Binding Proteins Influence Filament Dynamics and Organization Monomer Availability Controls Actin Filament Assembly Actin-Nucleating Factors Accelerate Polymerization and Generate Branched or Straight Filaments Actin-Nucleating Factors Accelerate Polymerization and Generate Branched or Straight Filaments Actin-Nucleating Factors Accelerate Polymerization and Generate Branched or Straight Filaments Higher-Order Actin FilAment ArrAys InFluence CellulAr MechAnicAl Properties And SignAling Higher-Order Actin Filament Arrays Influence Cellular Mechanical Properties and Signaling The Functions of Actin Filaments Are Inhibited by Both Polymer-stabilizing and Polymer-destabilizing Chemicals • The Functions oF Actin FilAments Are inhibited by both polymer-stAbilizing And polymer-destabilizing chemicals Chemotherapy medication Gout treatment BacteriA CAn HijAck the Host Actin Cytoskeleton The Actin-based movement of Listeria monocytogenes Bacteria Can Hijack the Host Actin Cytoskeleton The Actin-based movement of Listeria monocytogenes ACTIN AND ACTIN-BINDING PROTEINS SUMMARY • Actin-binding proteins influence filament dynamics and organization. • Monomer availability controls actin filament assembly. • Actin-nucleating factors accelerate polymerization and generate branched or straight filaments. • Actin filament-binding proteins alter filament dynamics. • Severing proteins regulate actin filament depolymerization. • Higher-order actin filament arrays influence cellular mechanical properties and signaling. • Bacteria can hijack the host actin cytoskeleton. MYOSIN AND ACTIN • Actin-based motor proteins are members of the myosin superfamily. • Myosin generates mechanical force by coupling ATP hydrolysis to conformational changes. • Myosin II is the first motor protein discovered in muscle. Sliding of myosin II along actin filaments causes muscles to contract. • Myosin V binds to vesicular cargo to transport it along actin filaments. Table 12.2 Some Types of Myosins and Their Functions Molecular Weight of Type Main Function Heavy Chain (kDa) I 110-150 MembrAne binding II 220 FilAment sliding in muscle V 170-220 Vesicle trAnsport TrAnsport oF endocytic vesicles, VI 140 moves towArds the “-” end oF the actin Fiber Textbook of Structural Biology 38 Myosin superfamily members MyosinFigure and 17.31 Muscle Structure Function of the skeletal- Muscle muscle sarcomere. Architecture Myofibrils: bundles of contractile protein fibers Sarcomere: Z-disc Thin filament: actin fiber Thick filament: 300 myosin molecules M-band (M-line) A-band I-band MOLECULAR CELL BIOLOGY SIXTH EDITION 40 Sliding of Myosin II Along Actin Filaments Causes Muscles to Contract AssociAting protein- Titin: A lArge And long protein (2.7MDA); extend From M-band to Z-disc to maintain the length of the sarcomere. Actin-Based Motor Proteins Are Members of the Myosin Superfamily- Myosin II Actin-Based Motor Proteins Are Members of the Myosin Superfamily 300 myosin molecules Source: Structural Biology Myosin Generates Force by Coupling ATP Hydrolysis to Conformational Changes MYOSIN AND ACTIN 1. Muscle contraction Striated muscle cells Smooth muscle cells Cardiac muscle cells 2. Contractile bundles in nonmuscle cells are less organized. It is regulated by myosin phosphorylation. Examples: adherens belt in epithelial cell stress fibers in migrating cells contractile ring during cytokinesis 45 Myosin V carries cargo along actin filaments.
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