CYTO SKELETON FC

What is the Cytoskeleton

a network of proteins fibres in the cell that keep it structured.

• microtubules for transport (railway tracks)

• actin for movement and shape

• intermediate filaments for strength

Microtubules

allow for transport throughout the cell

• vescicles

• mitochondria

• chromosomes

form cilia and Flagella

form the spindle fibres

What are microtubules made from

alpha tubulin

beta tubulin

these are joined together to form a tubulin dimer

many of them added togehter = microtubulin

Why are microtubulin tubes

alpha nd beta tubulin bind together → homodimer → many homodimers rows → protofilament → 13 protofilament arranged in a circle → microtubule

Microtubules are strong:

• 13 protofilamnets

• hollow tube

• resistant to bending

microtubulin polarity

have a (+) and a (-) side but NOT CHARGED

simply a direction so the stuff incside cell knows where to go

(-) end is normally anchored inside the cell at the centrosome

(+) end usually grows and shrinks

Dynamic Instability

microtubules are awlways growing and shrinking and regrowing

Why do mIcrotubulin grow?

Every tubulin dimer has a GTP

Tubulin + GTP → joined the microtuble → GTP becomes GDP

GDP is unstable until a new GTP + tubulin arrive

• this forms a new GTP cap until GTP turns into GDP and the cycle continues = growth

Catastrophe and Rescue

GTP cap stabilises the plus end of microtubulin

loss of cap → rapid depolymerisation → catasprophe

new GTP cap forms as net GTP Tubulin bind → rescue

causes growth

Microtubulin Associated Proteins

Cells that control microtubules

• Tau

  • ties neighbouring microtubules together = stability and prevents collapse

  • in neurons important because → axons are so long → no stability means transport would fail

Kinesin - 13

active on the (+) side and causes catastrophe → de growth

used in mitosis because spindle fibres have to shorten

motor protein and travels on the microtubule

Motor Proteins

like the train and moves on the microtubules

2 types:

Kinesin

• moves towards the (+) end

Dynein

• moves towards the (-)

motor proteins need ATP

Actin Structure

Made up of G actin → polymerise to F - Actin

• makes it polar but no charge just the direction

• makes it flexible

Actin and ATP

actin travels

ATP hydrolysis powers actin to move

• Actin monomers add ATP bound actin to the filament on the (+)

• once added to filament → hydrolysed into ADP

• ADP → less stable

Treadmilling?

Acton has both ends exposed

Subunits are added at the (+) end

• Add ATP bound G actin

and removed at the (-) end.

• ATP is hydrolysed → ADP

actin is the same size and doesnt increase in length but moves like a treadmill

• Free G actin monomers bound to ATP are added to the (+) end of the actin filament

• once added the intrinsic ATPase activity of Actin → hydrolyses the bound ATP into ADP and Pi

• the older regions of the filaments near the (-) become more ADP bound.

Microtubule and Actin Compare

Microtubules

• Grow the Collapse → Dynamic Instabiility

• one side (-) is fixed at centrosome and the (+) grows and shrinks

Actin

• same length throughout

• Treadmilling

• (+) gets added Actin bound ATP then the (-) ATP hydrolysed to ADP

Actin Binding Proteins

Profilin

• adds actin

• promote polymerisation

Thymyosin

• stores actin away

• inhibits polymerisation

Actin shape.

Actin can be branched or straight

Formins:

• build straight actin

• used for stress fibres and microvilli

• Rho activated formins → straight brnaches

Arp2/3

• creates branches

• Rac activates Arp2/3 → branched actin

Myosin

binds to actin and produces a contraction

• contractions in the muscles, stress fibres and cytokenesis (pulls the nuclear envelop apart)

Actin Function

• maintain cell shape

• form microvilli

• allows migration

• helps cell divide

Actin Filaments

Move

Intermediate Filaments

DONT MOVE

• for strength

• NO GTP

• NO ATP

like ropes holding the cell down

made of keratin → many disulfide bonds = strong

Lamins

type of intermediate filament inside the nucleus

• support the nucleus

• organise chromatin

• connect nucleus to the cytoskeleton

lamins during mitosis

enzyme CDK1 phosphorylates lamins

phosporylation causes lamins to depolymerise → reduce in size → less strength in the nuclear envelope so it can be pulled

Lamin Related Disease

Epidermolyisis Bullosa

• mutation in keratin

• weak skin

• blistering with minor friction

Muscular Dystrophy

• fragile nuclear envelope

• muscle weakness and damage

Why epithelial cells have high mechanical strength

Intermediate filaments attach to

Desmosomes: Join cell to cell

Hemidesmosomes: join cell to basement membrane

in epethelial cells

Cyoskeleton in Epithelial cells (A whole cell)

Microtubules

• organise polarity and direct vesicles to the correct membrane surface

Actin

• forms microvilli = increases surface area for absorption

Intermediate Filaments

• provide strength by linking desmosomes and hemidesmosomes

CRISPR gene knockout

can be used to determine protein function

• remove a gene and see what happens to the cell

• e.g. remove Tau → axonal microtubules become unstable → tau stabilises the microtubules

Fluorescent Microscopy

• can be used to determine protein function

• Tag proteins and see where they interact in cells

• tag with GTP

Immunoprecipitation

can be used to see which proteins interact with each other

Co-Immunoprecipitation