The Cytoskeleton
- 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
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
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
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
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
Cytokinesis
In cell division - cleavage furrow constists of contractile ring
Made of actin filaments w/ interfering myosin
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
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)
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
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
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
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
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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