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

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

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

Nucleation Is the

Rate-limiting

Step in the

Formation of a

Cytoskeletal

Polymer

The Two Ends of

Microtubules

and Actin

Filaments Are

Distinct and Grow at Different

Rates

Filament Treadmilling and Dynamic Instability Are

Consequences of Nucleotide Hydrolysis

Tubulin -

dynamic

instability

Intermediate

Filaments

Impart

Mechanical

Stability to

Animal Cells

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

HOW CELLS REGULATE THEIR

CYTOSKELETAL FILAMENTS

• Microtubules Are Nucleated by a Protein Complex Containing γγγγ-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

Microtubules Are Nucleated by a

Protein Complex Containing γγγγ-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

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

Actin-based Motor Proteins Are

Members of the Myosin Superfamily

• Myosin II is the

muscle motor

• Other myosins

have other

functions

There Are Two Types of Microtubule

Motor Proteins: Kinesins and Dyneins

Motor Proteins

Generate Force by

Coupling ATP

Hydrolysis to

Conformational

Changes

Different motors can run in

opposite directions

Cilia and Flagella Are Motile Structures

Built from Microtubules and Dyneins

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

Mechanisms of Cell Polarization Can Be Readily

Analyzed in Yeast Cells

Signal transduction pathway to

polymerization

Directional Assembly Dictates the Direction

of Cell Migration

The Complex Morphological Specialization of

Neurons Depends on The Cytoskeleton

CHAPTER 18 THE

MECHANICS OF CELL

DIVISION

• AN OVERVIEW OF M PHASE

• MITOSIS

• CYTOKINESIS

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

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

• 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

M Phase Depends on DNA Replication and Centrosome

Duplication in the Preceding Interphase

M Phase Is Traditionally Divided into Six Stages

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

Cytokinesis

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

Mitotic

Machinery

Microtubule Instability Increases Greatly

at M Phase

• MAPs stabilize

• Catastrophins destabilize

• Rapid turnover results in survival of only productive (capped, attached, stabilized) microtubules

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

Kinetochores Attach Chromosomes to the

Mitotic Spindle

Sister Chromatids Separate

Suddenly at Anaphase

Both Pushing

and Pulling

Forces

Contribute to

Anaphase B

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

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

Bacterial Fission - a

model for M phase

& FtsZ - a bacterial

tubulin homolog