Module 4 - The cytoskeleton and molecular motors

Question 1

Microtubules: what are they and what do their plus and minus sides do?

Answer 1

Unbranches cylinders of approximately 25nm diameter that assembled from tubulin heterodimers

Plus side - grows quickly with an exposed b-tubulin
Minus side - grows slowly if at all

Question 2

How are microtubules made within cells/within test-tubes

Answer 2

Tubulin concentration is too low for spontaneous polymerisation so cells use a template of gamma-tubulin and other proteins (nucleation)

Tubulin, GTP, and Mg²⁺ left at 37°C can manufacture microtubules

Question 3

How do microtubules grow?

Answer 3

Grow at the plus end of the tubulin ring complexes of centrosomes

Question 4

Microtubles

Answer 4

They are dynamic and grow and shrink independently of its neighbours

Question 5

What does slow hydrolysis give rise to?

Answer 5

On/off switches as GDP dissociating is a slow process and GTP associating is a fast process

Question 6

Why does tubulin need GTP?

Answer 6

GTP tubulin can polymerise with other dimers, meanwhile, GDP tubulin cannot

Question 7

How do microtubules have dynamic instability?

Answer 7

The GTP cap on the minus end of the microtubule will determine whether growth or shrinkage occurs (no GTP cap - shrinkage)

Question 8

How can microtubules be stabilised?

Answer 8

By the binding of microtubule-associated proteins (MAPs) along the microtubule, drugs (taxol), or capping complexes (proteins at the cell membrane attach to the microtubules and stabilise them)

Question 9

Can microtubules be depolymerised?

Answer 9

Yes, by putting cells on ice or using drugs that bind free tubulin dimers (Nocodazole, Colchicine, and Colcemid)

Question 10

Actin filaments: Subunit, filaments, drugs, and motor proteins?

Answer 10

Monomeric actin, binding ATP

2-stranded 7nm flexible helix

Phalloid (stabilises filaments), Cytochalasin (caps polymerisation from ends), and Latrunculin (binds to actin filaments, preventing polymerisation)

Myosin

Question 11

Microtubules filaments: Subunit, filaments, drugs, and motor proteins?

Answer 11

Tubulin dimers, binding GTP

Fairly rigid hollow 25nm tube

Taxol (stabilising), Nocodazole (destabilising)

Dyneins, kinesins

Question 12

Where are actin filaments found?

Answer 12

Sarcomeres, contractile fibres in other cells (dividing cells, stress fibres), and are found in some non-contractile parts of cells (microvilli)

Question 13

Actin polymerisation: how does it work?

Answer 13

The plus end of the actin filament is built up by ATP-bound actin monomers and the minus end of the filament loses the ADP-bound actin filaments (this allows the actin filament to almost always have the energy bound for use(?))

Question 14

Actin filaments: can they be stabilised and how would experimental manufacture of actin filaments work?

Answer 14

Yes, by a protein cap attaching

Actin, ATP, and Mg²⁺ left at 37°C can manufacture microtubules

Question 15

Why does actin not immediately polymerise and form filaments in the cell?

Answer 15

5% of the cell’s proteins are actin. In a test tube, they would all bind and form actin filaments; however, many proteins in the cells prevent this from occurring until the actin filaments are required

Question 16

Lamellipodia: where is it located and how does actin interact with it?

Answer 16

The end of the cell that is moving in that direction

Actin polymerises at the lamellipodia and causes an increase in length, causing the other side to move, beginning contraction

Question 17

Arp: what is it and what does it do?

Answer 17

Arp 2/3 complex -

Actin-related proteins which bind to existing actin sites, nucleating actin assembly, and preventing the minus-end disassembly

Question 18

Filopodia: what are they and what do they do?

Answer 18

The protrusions at the leading edge of migrating cells, extended by actin polymerisation at the cell membrane that are anchored by proteins

They guide the cell by probing the environment and reacting with surrounding ECM

Question 19

Mechanisms of animal cell migration

Answer 19

1 - Cell pushes out filopodia
2 - Cell adheres to surroundings
3 - contraction

Question 20

The adhering of animal cells

Answer 20

Focal contacts contain trans-membrane plasma proteins (integrins) as well as contractile actin bundles (stress fibres) adheres the cell

Question 21

Contraction of the cell

Answer 21

Myosin II (motor protein) causes movement

Question 22

Intermediate filaments: which organisms have them and what types are there?

Answer 22

Found in animals but not unicellular organisms, plants, or fungi

Cytoplasmic filaments
Nuclear filaments

Question 23

Cytoplasmic filaments: what are they, what animals have them present?

Answer 23

Keratin filaments - epithelial cells
Vimentin filaments - in connective cells
Neurofilaments - nerve cells

All animals except arthropoda and hydra

Question 24

Nuclear filaments: what are they, what animals have them present, and where are they located?

Answer 24

Nuclear laments

All animals

Just under the nuclear envelope, forming the nuclear lamina

Question 25

Progeria: what is it and how does it occur?

Answer 25

A premature ageing syndrome

Mutations in nuclear lamins

Question 26

Intermediate filaments: Subunit, filaments, drugs, and motor proteins?

Answer 26

Various that do not use ATP/GTP

8-stranded flexible helix (10nm)

None

N/A

Question 27

Properties of intermediate filaments

Answer 27

• Connected by desmosomes and strengthen cells
• Stable but may disassemble during replication (by phosphorylation)

Question 28

Keratin mutations: what do they cause?

Answer 28

In a keratin mutation, the cells rupture between the nucleus and hemidesmosomes and cause damage to underlying basal tissue, causing blistering

Question 29

Desmin mutations: where is desmin found and what do the mutations cause?

Answer 29

Desmin is found within every type of muscle and a mutation may lead to muscular dystrophy or cardiac myopathy

Question 30

Neurofilaments

Answer 30

Strengthen neurons

Question 31

Intermediate filaments assembly

Answer 31

Symmetrical filaments organised in an antiparallel manner which causes there to be no polarity

Question 32

What are the two main motor proteins and what movement do they do?

Answer 32

Kinesins - move cargo towards microtubule plus ends (cell periphery)

Dyneins - move cargo towards microtubule minus ends (cell centre)

Question 33

The two types of dyneins

Answer 33

Cytoplasmic dynein

Axonal dynein

Question 34

What are the differences between dyneins

Answer 34

All have the same dynein motor base, but each attaches to a different dynactin (adaptor protein)

Question 35

Viral entry into the cell

Answer 35

Enter using cytoplasmic dynein either within endosomes or as viral capsids

Question 36

Types of sliding

Answer 36

• Anti-parallel sliding (ie mitosis/meiosis)
• Parallel sliding (ie cilia/flagella)

Question 37

What are the axoneme structure in cilia/flagella and what purpose?

Answer 37

Ring of microtubule doublets with a two single microtubule centre with dynein arms attached

Used for cilial/flagellar beating

Question 38

Myosins: what uses them and what is the more well-known myosin?

Answer 38

Plants. algae and many Fungi use myosins along with actin filaments for transport

Myosin II is found in most cell types, not just muscle (also found in stress fibres)

Question 39

What allows myosin filaments to remain stable and what happens if they are not preset?

Answer 39

Desmin intermediate filaments form a scaffold which stabilises muscle Z discs

Muscle/cardiac dystrophy

Question 40

Myosin I: what does it do and what are its key features?

Answer 40

It helps reshape the membrane by pulling on underlying actin filaments and it also facilitates short organelle movement

Only has one head