The Cytoskeleton

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  • Created by: rosieevie
  • Created on: 19-05-17 15:45

Actin and Actin Filaments

Actin - most abundant protein in cells

Multiple genes code for actin

2 types:

  • Globular actin (G actin) - monomer 
  • Filamentous actin (F actin) - polymer of G actin

ATP binds to G actin in cleft - hydrolyses it into filament

F actin is polaried - monomers usually bind at +end and removed at - end

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Epithelial Cell Brush Border of Microvilli

Actin filaments regulated to form apical membrane 

  • Actin can bundle proteins together to regulate conformation
  • Fimbrin and billin crosslink actin filaments into bundles
  • Myosin 1 binds bundles to cell membranes
  • Spectrin - cross links actin bundles in microvilli actin rootlet
  • Microvillin actin complex sits on intermediate keratin filament platform
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Actin Polymerisation

G actin clumps together - forms nucleus of 3 or 4 monomers - slow process

More G actin rapidly joins to nucleus = filament elongation

Joining direction - dependent on concentration of G actin:

  • High concentration = actin joins at both ends
  • Critical concentration = actin joins at preferred + end

Treadmilling - G actin added and drops off end at steady state

Rate modified by actin-binding proteins - prevent binding

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Actin-Myosin Interaction

  • ATP binds to myosin head at rest
  • ATP hydrolysed - myosin head *****
  • Head binds to actin - Pi released
  • Power stroke moves filament
  • ADP released
  • Another ATP binds to head = rest position again
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Zonula Adherens Junction

Strong zonula adherens junctions hold together epithelial cells

Lie just under the tight junction

~15mm gap between epithelial cells

In the gap - 2 cadherin proteins which interact to 'zip' cells together

Inside the cell - actin and myosin filaments crosslink the adhesion junctions together - arranged in circle around cell (band)

Band can contract - keeps cell taught and can change shape

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Cytokinesis

In cell division - cleavage furrow constists of contractile ring

Made of actin filaments w/ interfering myosin

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Cell Locomotion

Cells move along plane as they are polarised

Receive signals regulating actin cytoskeleton = directionality

Myosin accumulates at back of cell = contraction of cytoplasm

Lamelpodium forms - short mesh of actin filaments that pushes membrane forward

New adhesion site formed further along

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Microtubule Organisation

Two types of tubulin:

  • a-tubulin - bind GTP in cleft
  • b-tubulin - hydrolise GTP to GDP and Pi

Microtubulin - spiral configuration of alternating a and b tubulin w/ hollow core and larger diameter than actin

Microtubule organising centre (MTOC) - microtubules emminate from here

Also have microtubule associated proteins (MAPs) - bind to tubules so dissociation cannot occur - controlled by different phosphorylation states

Need to rapidly reorganise for cell activity changes e.g. mitosis

Different cell types have different organisatione e.g. neurons - axons controlled by parallel microtubules (increases stability)

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Mitosis Microtubules

Cell undergoing mitosis has 3 sets undergoing dynamic instability:

  • Astral set - position spindles
  • Polar - lengthens to keep poles apart
  • Kinetochore MTs - contracts to draw spindles away from each other
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Dynamic Instability

Useful - causes instantaneous change in cell function

When tubulin added to microtubule its bound to GTP

GTP hydrolysed to GDP + Pi 

Growing tubule always has GTP cap

In low GTP concentration polymerisation rate of new GTP tubulin < hydrolysis rate = GDP cap replacing GTP

= rapid dissembley

MAPs detect and prevent dissembley

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Vesicle Transport

Vesicles need to be transported around cell or along cell's axon

Transported on 'tracks' of microtubules - attatched by ATPase enzymes using binding sites

ATPases contract and walk along tubules e.g. kinesin

Kinesin moves vesibles from - end to + end

Dyneins move vesicles from + end to - end

Organelle membranes have binding sites - can interact with ATPases and be moved into position = useful in rapid reorganisation during interphase

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Microtubule Organisation in Cilia and Flagella

In eukaryotes - 9 + 2 organisation

9 partially fused microtubules from outside

2 bridged microtubules in middle

Dynein binds to adjacent pairs of microtubules and contracts = controls flagella/cilia movement in power strokes

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Intermediate Filaments

Filaments not used in contraction but to stabalise cell organisation

Concentrate around the nucleus - form linkages w/ cell membrane, especially where anchorage to another cell occurs

Desmosomes - linkages w/ other cells by intermediate fillaments

Hemidesmosomes - linkages with extracellular matrix (increases stability of cellular layers)

Hemidesmosomes can destabalise so cells can migrate

Different cells types have different arrangements

Cytokeratin found in epithelial cells while desmin found in muscle cells

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