cytoskeletal protein - microtubule

8/12/2019 Cytoskeletal Protein - Microtubule

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Topic 12-1-1 1

Cytoskeleton Protein - Microtubule


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Eukaryotic cell Skeletal SystemThree well defined filamentous structures

Micotubules

Microfilaments

Intermediate filaments

Cytoskeleton


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Eukaryotic cell Skeletal System

Micotubules

Rigid tubes

Tubulin

Microfilaments

Solid / thinner

Actin

Intermediate filamentsTough ropelike fibres

Many related proteins

Cytoskeleton


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Topic 12-1-1 4

Functions of Cytoskeleton

1 Dynamic scaffold

2 Internal framework

3 Network of highways

4 Cell movement

5 m-RNA anchoring

6 Cell division

Cytoskeleton


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MicrotubulesStructure and Composition

Components of a diverse array of structures

Mitotic spindle

Core of flagella and cilia

Tubes of globular proteins

Longitudinal rows

Protofilaments

Cross-section13 rows of protofilamentscircular

Dimeric building blocksa-tubulin and b-tubulin

Cytoskeleton


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a-tubulin and b-tubulin

Similar 3-D structure

Form dimers

Fit togethernon-covalent bonds

Cytoskeleton


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a-tubulin and b-tubulin

Linear array

Asymmetric

a-tubulin at one end

b-tubulin at other endSame polarity

Plus end = fast growing

Minus end = slow growing

Cytoskeleton

+ -


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a-tubulin

Bound GTP

Not hydrolysed

Non-exchangable

b-tubulin

Bound GDP

Exchanged for GTP prior to assembly

Cytoskeleton


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Microtubule Associate Proteins (MAPs)

Mostly in brain

ExceptionMAP4many cell types (non-neuronal)

Domain attaches to microtubule

Domain extends outfilament

Various roles

Cross-bridges connecting microtubules

Increase microtubule stability

Alter microtubule rigidity

Alter microtubule rate of assemblyActivityphosphatases and phosphokinases

Cytoskeleton


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Microtubulesstructural roles

Determine cell shape

Axons of nerve cells

Internal organization

Axonal transportationMaterials moved from cell bodyalong axon

Anteriograde

From axon to cell bodyendocytosis

Retrograde

Axons have microfilaments, intermediate filamentsand microtubules

Interconnected

Cytoskeleton


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Microtubulesstructural roles

Passive

Tracks for many motor proteins

Motor proteins use ATP

Move cellular cargo

Vesicles, Mitochondria, Lysosomes, Chromosomes

Motor proteinsThree families

Myosins

Kinesins

Dyneins

Kinesins and Dyneinsmove on microtubules

Cytoskeleton


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Motor proteins

Move unidirectionally

Stepwise

Series of conformational changes

A mechanical cycle

Coupled to chemical cycleEnergy

Steps

ATP binding to motor

Hydrolysis of ATP

Release of ADP and Pi

Binding of new ATP

Cytoskeleton


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Motor proteins

KinesinTetramer

2 identical heavy and 2 identical light chains

Functional domains

Pair of globular heads

Bind microtubule

ATP-hydrolysing

Neck / stem and tail

Tail binds cargo

Move toward plus end of microtubule

Plus end directed

Cytoskeleton


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Motor proteins

Kinesin

Cytoskeleton


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Motor proteins

KinesinVelocity proportional to [ATP]

Move one heterodimer at a time (step)

One headalways attached

Heads are coordinated

Each at different stages of chemical and mechanical cycles

When one head binds

Conformational change in adjacent neck region

Swings other head forward

Kinesinwalks along microtubule

Cytoskeleton


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Motor proteins

KinesinOne member of a superfamily of related proteins

Kinesin related proteinsKRPs

Kinesin-like proteinsKLPs

> 50

Heads similar

Tails heterogenousbinding different cargoes

Cytoskeleton


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Motor proteins

Kinesin-mediated organelle transportKinesins aligned with plus ends away from nucleus

Tend to move organelles in anterograde direction

Cytoskeleton


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Motor proteins

Cytoplasmic DyneinMovement of cilia and flagella

And ubiquitous motor protein in eukaryotic cells

Huge - > 1.5 Mda

2 identical heavy chainsMany intermediate and light chains

Heavy chain

Large globular head

Force generating engine

Minus end directed

Cytoskeleton


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Motor proteins

Cytoplasmic DyneinTwo rolesForce generationspindlemitosis

Minus-end directed motor for Golgi Complex and vesicles

Requires a sub-unit complex dynactin

Cytoskeleton

cytoskeletal protein - microtubule

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