The Cell Cytoskeleton 1

The Cell Cytoskeleton Chapter 17

Questions in this chapter you should be able to answer:

Chapter 17: 1 - 11, 13 - 23

Watch this animationhttp://multimedia.mcb.harvard.edu/ anim_innerlife.html

The Inner Life of Cells

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All of these movements are made possible by the cell “Cytoskeleton”

What is the Cell Cytoskeleton?-- long protein filaments-- extend through the cytoplasm

A variety of purposes

Three families

•Microtubules•Actin filaments•Intermediate filaments

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The Cell Cytoskeleton

What are the 3 primary typesof cytoskeletal proteins?

Intermediate filaments-- resist mechanical stress

Microtubules-- cytoplasmic transport-- axoneme movement -- chromosome movement

Actin filaments-- membrane contraction

-- muscle cells-- cytokinesis

-- cell movements

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What is the structure of intermediate filamentsCable-like arrangement

Extended molecular interactions

Anchoring to desmosomes

Intermediate Filaments

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Intermediate filaments -- help hold cells & tissues together

Human lung carcinoma

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Intermediate filaments also occur in the nucleus

Cytoplasmic -- keratins – skin integrity -- in neurons -- in muscle, connective tidssue

Nuclear -- lamins

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Microtubules & Actin filaments make possible different types of movements

Cell migrationNeutrophil Flagella and cilia

Cytoplasmic movementsMuscle contractionMitosis and Cell divisionCytoplasmic transport

Membrane movements Membrane ruffling

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Helps to distribute materials through the cell

This highway is ‘dynamic’

Cytosolic Microtubules

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The ‘spindle apparatus’ controls chromosome movements

-- Attached to “kinetochores” on duplicated chromosome

Spindle ApparatusKinetochore function

Cell Division

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What is the fundamental structure of microtubules?

Alpha and beta tubulin subunits

13 member ring

Why do MTs have polarity?

‘MT Organizing Center’

Why are MTs said to display ‘Dynamic Instability’?

Dynamic Instability

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Kinesin walking

Motor proteins carry “cargo” along microtubules

Two motor proteins

Opposite directions

How does Kinesin ‘walk”?

Organelle movement

Kinesin walking

Organelle movement

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What is the mechanism of MT growth and retraction?

Binding, hydrolysis and release of GTP

‘Capping’ at cell membrane

Question 17-3, p 583

How would a change in [tubulin] affect MT dynamics?

..if only GDP were present?

… or a nonhydrolizable GTP analog?

MT with EB1 cap

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What is the structure and movement of an axoneme?

Cilia & flagella

“9 + 2 Structure”

Living Cell video

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Where are human cilia and flagella found?

Various places

Some genetic conditionscause cilia not to function

Symptoms of IMSRespiratory infection;

Fertility problems Etc…

and ‘situs inversus‘’ (50%)

Respiratory cilia

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An interesting congenital genetic disorder -- “Immotile Cilia Syndrome”

Symptoms:•Respiratory tract infections•Infertility•Digestive problems•Situs Inversus

-- Affects ~50% of people with IMS

- IDA - ODA

- IDA

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What is the link between cilia and Left/Right orientation?

Cilia occur in the “embryonic node”-- very early stage of development

Generate oriented flow of signal molecule

Why 50% affected?

Embryonic node cilia

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What is the distribution and structure of actin filaments?

treadmilling

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How are actin filaments organized in the cytoplasm?

The cell cortex

Actin binding proteins

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How does actin mediate cell movements?

Cell crawling

What are Lamellipodia and Filopodia?

Membrane ruffling

Listeria movement

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Actin Filaments are involved in muscle contraction

What happens when muscles contract?

Nerve impulses activate contraction at neuromuscular junction

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What is structure of muscle cells?

Some basic anatomy. . .

“Muscles” are bundles of muscle cells

Sarcomeres -- are the smallest contractile unit of muscles

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Some details of Sarcomeres,Actin and Myosin…

Nerve impulse triggers Ca++ release Inside of cell

-- Myosin pulls on actin-- Pulls sarcomere shorter

Ca++ quickly removed-- leads to relaxation

Muscle contraction

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Sarcomeres create visual ‘striations ‘of “Striated muscles” (skeletal muscles)

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Muscle spasms and cramps (“Charley Horse”)

Imbalance in excitatory and inhibitory signals-- self-reinforcing cycle occurs

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Myosin is attached

ATP binds -- Myosin head detaches

ATP is hydrolyzed -- Myosin Head cocks -- loosely binds to actin

Pi is released -- which triggers ….

Powerstroke -- during which…ADP is released

Myosin remains attached

How does the sliding filament model explain sarcomere contraction?

myosin

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How is muscle contraction triggered?

Motor neuron & action potential

Sarcoplasmic reticulum

T-tubules

Ca++ release

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How does Ca++ trigger the muscle contraction?

The tropomyosin / troponin complex

Sarcomere

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Question

In order to keep cytosolic Ca++ levels low, muscle cells possess an ATP driven Ca++ pump in the sarcoplasmic reticulum and a Ca++/Na+ ATPase in the cell membrane. The cells also possess the Na+/K+ ATPase in the cell membrane. The Na+/K+ ATPase is partially inhibited by drugs such as ouabain and digitalis, whereas the Ca++/Na+ ATPase is inhibited by binding to a protein called phospholamban.

A. Draw a diagram showing the expected arrangement and orientation in the membranes of these membrane proteins.

B. Would treating a patient with either of these drugs weaken or strengthen muscle contraction (they are usually given to cardiac patients)? Explain.

C. The regulatory protein “protein kinase C” (PKC) regulates activity of the Ca+

+ ATPase. PKC can phosphorylate (covalently add a PO4) to the Ca++ ATPase, which increases its affinity for Ca++. What would be the expected effect of Ca++ ATPase phosphorylation on the strength of muscle contraction?