CHAPTER 16 THE CYTOSKELETON

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1 CHAPTER 16 THE CYTOSKELETON THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS HOW CELLS REGULATE THEIR CYTOSKELETAL FILAMENTS MOLECULAR MOTORS THE CYTOSKELETON AND CELL BEHAVIOR

2 THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS Three Types of Cytoskeletal Filaments Nucleation Is the Rate-limiting Step in the Formation of a Cytoskeletal Polymer The Two Ends of a Microtubule and of an Actin Filament Are Distinct and Grow at Different Rates Filament Treadmilling and Dynamic Instability Are Consequences of Nucleotide Hydrolysis by Tubulin and Actin Intermediate Filaments Impart Mechanical Stability to Animal Cells Filament Polymerization Can Be Altered by Drugs

3 Three Types of Cytoskeletal Filaments Actin filaments (5-9 nm diameter) Actin subunits Locomotion, muscle contraction Intermediate filaments (10 nm diameter) Various coiled coil protein subunits (lamins, vimentin, keratin) Structural roles Microtubules (25 nm diameter) Tubulin subunits Intracellular transport

4 Nucleation Is the Rate-limiting Step in the Formation of a Cytoskeletal Polymer

5 The Two Ends of Microtubules and Actin Filaments Are Distinct and Grow at Different Rates

6 Filament Treadmilling and Dynamic Instability Are Consequences of Nucleotide Hydrolysis

7 Tubulin - dynamic instability

8 Intermediate Filaments Impart Mechanical Stability to Animal Cells

9 Filament Polymerization Can Be Altered by Drugs TABLE 16 2 Drugs That Affect Actin Filaments and Microtubules ACTIN-SPECIFIC DRUGS Phalloidin binds and stabilizes filaments Cytochalasin caps filament plus ends Swinholide severs filaments Latrunculin binds subunits and prevents their polymerization MICROTUBULE-SPECIFIC DRUGS Taxol binds and stabilizes microtubules Colchicine, colcemid binds subunits and prevents their polymerization Vinblastine, vincristine binds subunits and prevents their polymerization Nocodazole binds subunits and prevents their polymerization

10 HOW CELLS REGULATE THEIR CYTOSKELETAL FILAMENTS Microtubules Are Nucleated by a Protein Complex Containing γ-tubulin in the Centrosomes of Animal Cells Regulatory Proteins Bind to Free Subunits, Filaments Sides and Filament Ends Extracellular Signals Can Induce Major Cytoskeletal Rearrangements

11 Microtubules Are Nucleated by a Protein Complex Containing γ-tubulin in the Centrosomes of Animal Cells

12 Regulatory Proteins Bind to Free Subunits, Filaments Sides and Filament Ends

13 Extracellular Signals Can Induce Major Cytoskeletal Rearrangements

14 MOLECULAR MOTORS Actin-based Motor Proteins Are Members of the Myosin Superfamily There Are Two Types of Microtubule Motor Proteins: Kinesins and Dyneins Motor Proteins Generate Force by Coupling ATP Hydrolysis to Conformational Changes Cilia and Flagella Are Motile Structures Built from Microtubules and Dyneins

15 Actin-based Motor Proteins Are Members of the Myosin Superfamily Myosin II is the muscle motor Other myosins have other functions

16 There Are Two Types of Microtubule Motor Proteins: Kinesins and Dyneins

17 Motor Proteins Generate Force by Coupling ATP Hydrolysis to Conformational Changes

18 Different motors can run in opposite directions

19 Cilia and Flagella Are Motile Structures Built from Microtubules and Dyneins

20 THE CYTOSKELETON AND CELL BEHAVIOR Mechanisms of Cell Polarization Can Be Readily Analyzed in Yeast Cells Directional Assembly Dictates the Direction of Cell Migration The Complex Morphological Specialization of Neurons Depends on The Cytoskeleton

21 Mechanisms of Cell Polarization Can Be Readily Analyzed in Yeast Cells

22 Signal transduction pathway to polymerization

23 Directional Assembly Dictates the Direction of Cell Migration

24 The Complex Morphological Specialization of Neurons Depends on The Cytoskeleton

25 CHAPTER 18 THE MECHANICS OF CELL DIVISION AN OVERVIEW OF M PHASE MITOSIS CYTOKINESIS

26 AN OVERVIEW OF M PHASE Cohesins and Condensins Help Configure Replicated Chromosomes for Segregation The Cytoskeleton Carries Out Both Mitosis and Cytokinesis Two Mechanisms Help Ensure That Mitosis Always Precedes Cytokinesis M Phase Depends on DNA Replication and Centrosome Duplication in the Preceding Interphase M Phase Is Traditionally Divided into Six Stages

27 Cohesins and Condensins Help Configure Replicated Chromosomes for Segregation

28 The Cytoskeleton Carries Out Both Mitosis and Cytokinesis

29 Two Mechanisms Help Ensure That Mitosis Always Precedes Cytokinesis 1. Proteins required for cytokinesis are inactivated by M-Cdk during mitosis 2. The remnants of the mitotic spindle are required for assembly of the contractile ring before cytokinesis

30 M Phase Depends on DNA Replication and Centrosome Duplication in the Preceding Interphase

31 M Phase Is Traditionally Divided into Six Stages

32 Prophase

33 Prometaphase

34 Metaphase

35 Anaphase

36 Telophase

37 Cytokinesis

38 MITOSIS Microtubule Instability Increases Greatly at M Phase Interactions Between Opposing Motor Proteins and Microtubules of Opposite Polarity Drive Spindle Assembly Kinetochores Attach Chromosomes to the Mitotic Spindle Anaphase Is Delayed Until All Chromosomes Are Positioned at the Metaphase Plate Sister Chromatids Separate Suddenly at Anaphase Kinetochore Microtubules Disassemble at Both Ends During Anaphase A Both Pushing and Pulling Forces Contribute to Anaphase B At Telophase, the Nuclear Envelope Re-forms Around Individual Chromosomes

39 Mitotic Machinery

40 Microtubule Instability Increases Greatly at M Phase MAPs stabilize Catastrophins destabilize Rapid turnover results in survival of only productive (capped, attached, stabilized) microtubules

41 Interactions Between Opposing Motor Proteins and Microtubules of Opposite Polarity Drive Spindle Assembly (-) end motors (like Kar3p) organize tubules at spindle poles (+) end motors (like Cin8p) push tubules of opposite orientation against each other

42 Kinetochores Attach Chromosomes to the Mitotic Spindle

43 Sister Chromatids Separate Suddenly at Anaphase

44 Both Pushing and Pulling Forces Contribute to Anaphase B

45 CYTOKINESIS The Microtubules of the Mitotic Spindle Determine the Plane of Animal Cell Division Actin and Myosin II in the Contractile Ring Generate the Force for Cytokinesis Membrane-enclosed Organelles Must Be Distributed to Daughter Cells During Cytokinesis Mitosis Can Occur Without Cytokinesis The Phragmoplast Guides Cytokinesis in Higher Plants The Elaborate M Phase of Higher Organisms Evolved Gradually from Procaryotic Fission Mechanisms

46 The Microtubules of the Mitotic Spindle Determine the Plane of Animal Cell Division

47 Actin and Myosin II in the Contractile Ring Generate the Force for Cytokinesis

48 Bacterial Fission - a model for M phase & FtsZ - a bacterial tubulin homolog