Week 8 - Cytoskeleton Flashcards
What is the definition of the cytoskeleton?
a large dynamic netwrok of protein filaments which support the cell
What are the functions of the cytoskeleton?
1 - Shape
2 - Strength
3 - Movement
What are the 3 types of protein filament?
1 - actin filaments
2 - microtubules
3 - intermediate filaments
Function of the actin filament
shape of the cells surface
whole cell locomotion
pinching one cell into two
Function of the microtubules
position of the organelle
intracellular transport
mitotic spindle
Function of the intermediate filaments
mechanical strength
What are actin filaments?
- formed by the polymerization of globular actin monomers
- non covalent bonds between adjacent molecules
- the monomers form a protofilament
- two protofilaments twist to form a helix (F-actin)
What is the F-actin structure?
- polymer chains form a double helix
- G-actin is the monomer from which F-actin is formed
- linear bundles, 2D networks, 3D gels
- has a large cleft that binds to ATP
- due to polarity all G actin subunits are pointing the same way
What is nucleation?
the first step in formation of a new structure via self assembly
2 ATP-G-actin subunits bind to each other weakly
when 3 ATP-G-actin subunits form a trimer its acts as a nucleus from which a larger filmament can form
What is tread milling?
subunits added much faster at the positive end
subunits dissociate much faster at the negative end
causing the positibve end to grow and the negative end to shrink
the overall length stays constant however
this is essential for cell mobility and alterations in cell shape
Name 4 biological phenomenas that are aided with cell migration
1 - embryogenesis
2 - inflammatory response
3 - woound healing
4 - metastasis
What are the two shapes cause by moving actin filaments?
1 - lamelipodia - branched network of actin filaments
2 - filopodia - bundled network of actin filaments
all treadmilling to move forward
What are responsible for contractile force?
stress fibres - they spread and stretch when moving
What are microtubules?
- formed by the polymerisation of tubuline heterodimers consisting of an alpha and beta tubulin
- non covalent bonds between the alpha and beta tubulins
- tubulin subunits form protofilaments
- 13 protofilaments form a long hollow cylinder with a central lumen which is essental for intracellular trafficking
What is the structure of microtubules?
- largest filament
- each protofilament is formed of repeating alphabeta tunulin hereodimers
- 13 longitudually repeating units of protofilaments
- slightly offset giving a helical structure
- vertical interaction between opposite tubulins, lateral interactions between same tubulins
- they are very stiff
What is the polarity in microtubules?
- beta exposed at positive end
- alpha exposed at negative end
- GGTP binding pocket in both domains but is only hydrolysed in beta tubulin subunit
What is microtubules dynamic instability?
The dynamics are driven by the rate of tubulin addition and rate of GTP hydrolysis in the beta tubulin
the dissociation of tunulin GDP is favourable
Tend to extend and shrink from the plus end as the negative end is permantely stabilised at the MTOC
What is the MTOC?
It is called the centrosome
this is where the negative end is anchored to
What does the centrosome consist of?
1 - a pair of centrioles
2 - pericentriolar material (matrix of proteins) surrounding
What are centrioles?
short, hollow cylinders of microtubules
orientated at 90 degrees to one another
What is the pericentriolar material?
most important component in the synthesis of microtubules
- amorphous material
- contains the gamma tubulin ring complex
- this consists of multiples copies of gamma tubulin complexed with numerous other proteins
- it is the specific site of nucleation for microtubules
Intracellular trafficking with microtubules
- microtubules have associated motor proteins
- unlike actin it has 2 associated motor proteins responsible for movement
1 - kinesin
2 - dynein
What does the motor protein kinesin do?
moves towards the positive end of the microtubule
ANTEROGRADE (away from centre)
What does the motor protein dynesin do?
moves towards the negative end of the microtubule
RETROGRADE (towards the centre)
What is the process of intracellular trafficking?
1 - Lagging head is ATP bound:strongly associated with MT. Leading head is ADP bound:weakly associated with MT
2 - Hydrolysis of ATP causes the lagging head to dissociate from MT
3 - Leading bind binds to ATP causes a conformation change
4 - lagging head then moves forwards
5 - cargo gets transported along MT
this all happens on the MT surface and on the kinesin molecule
What are intermediate filaments?
- much greater tensile strength then the other 2
- non polar!
- no associated motors therefore not involved in movement
- great heterogeneoty within intermediate filmaments
What is the structure of intermediate filaments?
32 monomers form one filmament
monomers are of alpha helical region of fibrous proteins
