Cytoskeleton

Question 1

What is definition of cytoskeleton?

Answer 1

Skeleton of cell

Question 2

What does the cytoskeleton provide for?

Answer 2

Shaping of cell
Intracellular movement of organelles
Cell movement

Question 3

What are 3 different polymers that make up cytoskeleton?

Answer 3

Microtubules
Intermediate filaments
Actin filaments

Question 4

Describe the structure of microfilament(actin filament)

Answer 4

Helical polymers made of actin
Flexible, organised into 2D networks +3D gels
Are most highly concentrated in cortex just beneath the plasma membrane
Twisted chain of units of protein actin, G actin
Chain constitutes filamentous form (F actin)
Prevents structural polarity
Associated with larger number of actin binding proteins (ABP)
There are 3 isoforms of G actin with different isoelectric point=> a actin found in mainly in muscle cells, B actin + gamma actin in non-muscle cells

Question 5

Describe the structure of intermediate filament?

Answer 5

Heterogenous group of filamentous proteins
Rope like structure with many long strands twisted together + made up of different subunits
Toughest of cytoskeleton filaments
Intermediate size between actin + microtubules
Form network => throughout the cytoplasm, joining up to cell-cell junctions (desmosomes) + surrounding nucleus

Question 6

Describe the structure of microtubules?

Answer 6

Hollow tubes made of protein Tubulin 
Rigid, long straight 
Thickest of the filaments 
Each filament is polarised 
It is dynamic structure => assemble + dissemble in response to cell needs

Question 7

What is function of microfilament?

Answer 7

Cell shape
Organelle shape
Cell migration

Question 8

What is function of intermediate filaments?

Answer 8

Mechanical strength=> resists high pressure

Give the shape of the cell

Question 9

What is function of microtubules?

Answer 9

Organelle positioning

Intracellular transport=> facilitate the distribution of components through cell

Question 10

What are 2 actin binding proteins that control G actin levels?

Answer 10

Profilin: facilitates actin polymerisation-> binding of monomers to filament therefore to extend filament. Having actin monomer bound to profilin helps the reaction
Thymosin B4: prevents addition of actin monomers to F actin. Thymosin B4 competes with profilin + binds to actin to form actin+thymosin complex=> cannot bind to actin filaments

Question 11

What is function of cross-linking protein?

Answer 11

Maintains F actin in gel-like meshwork

Question 12

What is function of actin bundling protein?

Answer 12

Form thicker bundle of actin

Keep F actin in parallel bundles

Question 13

What is function of F actin severing protein?

Answer 13

Break F actin into smaller filaments

By severing them, there are more ends where actin can be depolymerised=> it accelerates the process

Question 14

What is function of motor protein (myosin)?

Answer 14

Transport of vesicles and/or organelles through actin filaments

Question 15

Examples of microtubules

Answer 15

Cilia in respiratory tract, sweeping mucus + debris from lungs
Flagella on spermatozoa

Question 16

What are intermediate filaments binding proteins?

Answer 16

Mainly linker of IF structures

IFBP stabilise + reinforce IF into 3D structure

Question 17

Give examples of IFBP

Answer 17

Fillagrin=> binds to keratin filaments into bundles
Synamim + Plectin => bind desmin + vimentin, link IF to other cytoskeleton compounds as well as to cell-cell contact structure (desmosomes)
Plakins => keep contract between desmosomes of epithelial cells

Question 18

Actin polymerisation

Answer 18

ABP= actin binding proteins 
Actin filaments (F actin) can grow by addition of actin monomers (G actin) at either end 
Length of filament determined by:
Concentration of G actin 
Presence of ABP

Question 19

Intermediate filaments polymerisation

Answer 19

Each unit made of
N terminal globular head
C terminal globular tail
Central elongated rod-like domains

Units form stable dimers
Every 2 dimers form tetramer => bind to each other + twist to constitute rope-like filament

Question 20

Microtubules polymerisation

Answer 20

Microtubule organising centre (MTOC) re specialised protein complexes from where assembly of tubulin unit starts
Centrosome is MTOC in most of cells=> contains gamma tubulin ring that initiates microtubule growth
Heterodimers of a +B tubulin constitute microtubule
It is polarised growth