The Cytoskeleton

Question 1

What is the cytoskeleton?

Answer 1

A complex network of protein filaments and tubules that extend throughout the cytoplasm to provide:

• structural integrity
• organisation
• stability

Question 2

Describe the structure of actin.

Answer 2

There are 2 forms: 
- G-actin (monomers)
- F-actin (filament)
(a twisted chain of G-actin monomers is 7nm in width)
It is isoelectric.
There are 3 isoforms of actin filaments:
- Alpha - found in muscle cells
- Beta - found in non-muscle  cells
- Gamma - found in non-muscle cells

Question 3

What are the functions of the 6 actin binding proteins?

Answer 3

1) Profilin - increases the rate of polymerisation
2) Thymosin beta 4: decreases the rate of addition, causing the filament to decrease in size
3) Actin bundling proteins: keep F-actin in a parallel arrangement
4) Cross linking proteins: keep the F actin in a gel like mesh work
5) F-actin severing proteins: breakdown F-actin into smaller filaments
6) Motor proteins (myosin): transport of vesicles/organelles through actin filaments

Question 4

What are the functions of F-actin?

Answer 4

IN SKELETAL MUSCLE: interaction with myosin motors allows for contraction
IN NON MUSCLE: contractile bundles (to aid movement of cell), lamellipodia and filopodia (protrusions for cell movement), contractile ring (that cuts the diving cells in two)

IN CELL MIGRATION:

1) Elongation - protrusions (lamellipodia and filopodia) pushed out
2) Adhesion - Integrins link the filaments to the extracellular matrix
3) Contraction - actin and myosin interaction, contraction and retraction

Question 5

Describe the structure of intermediate filaments.

Answer 5

It has an N terminal (amine) and a C terminal (carboxyl). The middle is a rod-like domain.
Normally, it is about 8-12nm wide.
Two filaments make a dimer.
Two dimers make a tetramer.
Multiple tetramers bind to make a rope-like filament unit.
It is the toughest filament (cyoplasmic being the most stable)

Question 6

What are the types of intermediate filaments?

Answer 6

CYTOPLASMIC:

1) Keratins (in epithelia, protects from damage/stress)
2) Vimentin/ vimentin related (in connective tissues, muscle cells and neuroglial cells)
3) Neurofilaments (in nerve cells)

NUCLEAR:
1) Nuclear lamins (in all nucleated cells)

Question 7

What are the functions of the 3 intermediate filament binding proteins?

Answer 7

1) Fillagrin: binds keratin into bundles
2) Synamin and Plectin: bind desmin and vimentin, and link IF to the filaments and microtubules
3) Plakins: keep contact between desmosomes

Question 8

What are the functions of intermediate filaments?

remember SOS

Answer 8

CYTOPLASMIC:

• Provides tensile strength, allowing it to withstand mechanical stress
• Create a 3D framework
• Fixing organelle positions

NUCLEAR:

• Form a mesh-like structure
• Lines the inner nucleus envelope to strength and provide attachment sites for chromatin
• Able to disassemble during cell division

Question 9

Describe the structure of microtubules.

Answer 9

They are hollow tubes made of tubulin. It is polarised, meaning it has a positive and negative end.
It is about 25nm wide. It is also relatively stiff.

Question 10

Describe the polymerisation of microtubules.

Answer 10

Microtubule organising centre (MTOC) are special centres where the assembly of tublin starts.
The centrosome (the MTOC in most cells) contains a gamma tubulin ring that initiates growth. Heterodimers of alpha and beta tubulin constitute the remainder of the growth (alternatingly)

Question 11

What are the functions of microtubules?

Answer 11

1) Intracellular transport:
They act like motorways for molecular motors. Directionality is important as motors can only travel in one direction.
2) Organises the position of organelles. This causes polarisation of cells (the polarisation refers to differences in shape, structure and function)
3) Rhythmic beating of cilia:
There are 9 microtubule pairs around an axenome (2 central microtubules)
Dynein is the motor protein that allows for bending of cilia and flagella.