L4 - Cytoskeleton

Question 1

Function of Cytoskeleton:

Answer 1

Gives structure to cells
Provides mechanical force allowing cells to change shape and move.
Moves organelles inside cells.
Required for cell division - separate chromosomes.

Question 2

Three types of cytoskeleton:

Answer 2

1. Actin Filaments (microfilaments) - thin
2. Microtubules - thick tubes
3. Intermediate filaments - not thin or thick

Question 3

Actin filaments:

Polarity:

Nucleation and Disassembly:

Answer 3

Actin Filaments:
Polymerises and depolymerises quickly.
Drive rapid changes in cell shape and movement.
Can produce force and movement with myosin.

Polarity:
Has barbed (+) end and pointed (-) end.
Barbed end has myosin and polymerisation occurs there.
Pointed end has no myosin. Is thin and depolymerisation occurs.
Treadmilling is constant and rely on ATP hydrolysis. - Filaments are same length. (Treadmilling - net addition at barbed end and net loss at pointed end). Maintains polarity as one end is different.

Nucleation and Disassembly:
ATP hydrolysis doesn’t drive polymerisation but is a timer. When Pi made it is released with actin monomer.
Actin can’t add to other monomers so bind to filament as formation of an actin trimer is rate limiting.

Question 4

Actin filaments: Different structures in cells

Answer 4

In Skeletal Muscle cells, there are parallel array of actin and myosin filaments. Have dark and light regions and are parallel (filaments).

In Mesenchymal cells (fibroblasts etc), there are:

1. Lamellipodia - branched arrays where thin sheet at bottom of cell and branching network is complex network of actin filaments.
2. Filopodia - parallel bundles which have thread like structures. Thin bundle of actin filaments. Parallel so same direction. Barbed end near plasma membrane.
3. Stress Fibres - anti parallel bundles so run in different directions. Stable and strong structure.

Question 5

Actin filaments: Membrane Protrusion

Answer 5

Barbed end faces plasma membrane. Elongates (polymerise) at plus end pushes lamellipodium forward. Middle has stress fibres. Contraction at back mediated by myosin II pulling on bundles of actin filaments. Myosin induces contraction.

Question 6

Actin filaments: Regulation by actin-binding proteins

Answer 6

Actin polymerisation controlled by:
Actin monomer binding proteins - stop polymerising
Actin nucleators - stimulate formation of filaments
Actin filament elongation factors - allow elongation
Actin filament capping proteins - stop elongation
Actin filament severing proteins - cut filament in 2
Actin filament cross-linking proteins - keep them as bundle

Question 7

Arp2/3 Complex:

Answer 7

2 actin related proteins and 5 other ones.
Act as nucleator - nucleates branched array of filaments by binding to side of filament.
Important in lamellipodia.

Question 8

Myosin:

Answer 8

Myosin II is ATPase so hydrolyse ATP. Found in all cells.
Moves along actin filaments.
Head binds to AF and tail forms myosin filaments.
Is a thick filament in muscle - muscle contraction.
Myosin pulls actin towards m line so muscle shorter as Z disc closer so contracts.

Question 9

Microtubules:

Function:

Structure:

Answer 9

Function:
Move organelles and vesicles.
Required for mitosis - move chromosomes.
Provide motility for flagella and cilia - allow rotation.

Structure:
Tube like and hollow.
Has dimers of α- and β-tubulin (which is at (+) end) both bound to GTP or GDP.
13 parallel protofilaments.
Polarity due to β-tubulin on plus end so polymerises faster.

Question 10

Microtubule-organising centre (MTOC):

Answer 10

MT cannot polymerise continuously in cell.
Nucleation of γ-tubullin rings in MTOC.
So MT grow and shrink and MTOC regulates how far they grow and shrink.

Question 11

Microtubules: Dynamic Instability

Answer 11

MT grow at (+) enf and rapidly shorts (catastrophe) and regrows. Hydrolysis of GTP bound to β-tubulin at (+) end drives this. When bound to GTP, they are attached to end of microtubule - forms GTP cap. GTP hydrolysed so tubulin bound GDP is released as it is unstable.

Centrosome nucleates MT and stabilises (-) end.
Plus end grows fast and depolymerises fast - catastrophe
Plus end of MT allows movement of chromosome and delivery of vesicles to plasma membrane.

Question 12

Cancer drugs that act of MT:

Answer 12

Drugs that stabilise MT and interfere with dynamic instability of mitotic spindle and block cell division:

1. Vinblastine - binds between tubulin dimers.
2. Taxol - Binds to β-tubulin.

Drugs that bind to tubulin dimers and inhibit assembly:

1. Colchicine
2. Nocodazole

Question 13

MT motor proteins:

Answer 13

Dynein move cargo to (-) end.
Kinesins move cargo to (+) end.
Both depend on hydrolysis of ATP.

Question 14

Intermediate filaments:

Functions:

Structure and Assembly:

Answer 14

Functions:
Flexible and Strong
Good tensile strength to cells and tissues.
Prevent excessive stretching of cells.
Not involved in intracellular movement of vesicles.
Reinforce axons of neurons.
Forms 2D mesh inside nucleus - gives spherical stability.

Structure:
Strong flexible, rope-like polymers
10nm in diameter
Desmosomes anchor them to adjacent cells - holds cells together e.g. skin (can stretch)
Stable inside cell and only depolymerises in cell division.

Question 15

Intermediate filaments: Diseases

Answer 15

Mutations in Keratin - blistering skin and brittle hair and nails.
Neuropathies - defects in connection. Learning deficiency.
Cardiomyopathy - heart problems.
Muscular Dystrophy - Cells don’t divide and decay over time.