Lecture 1 - Cytoskeleton I

Question 1

What is the Main Function of the Cytoskeleton?

Answer 1

1. Provide Cell Shape
2. Generation of Force

Question 2

What is the Structure of the Cytoskeleton?

Answer 2

Network of Proteinaceous Filaments (polymers) formed from individual subunits

Question 3

What are the properties of the Cytoskeleton and its Assembly?

Answer 3

• Self-assembling
• Dynamic (assembly/links to cellular components)

Question 4

State the Three Major Classes of Cytoskeletal Filament

Answer 4

• Actin
• Microtubules
• Intermediate Filaments

Question 5

What are Actin Microfilaments
(F-Actin)?

Answer 5

• “rope-like” filaments formed by 2 protofilaments of actin polymer intertwined

Question 6

What is the Structure of G-Actin?

What Enzymatic Activity does G-Actin Possess?

Answer 6

• Globular Protein containing a nucleotide-binding domain
• ATPase activity

Question 7

Describe the two ends of an actin microfilament

Answer 7

• Pointed End (-ve End) - slow growing
• Barbed End (+ve End) - fast growing

Question 8

Why is G-Actin present in an
ATP-bound state within cells?

Answer 8

Monomeric G-Actin is a poor GTPase

Question 9

How is the ATP.Actin Cap formed?

What is its purpose?

Answer 9

• Incorporation of new monomers faster than the ATP Hydrolysis of the previous monomer
• protects filament from depolymerisation

Question 10

Define the Basic Subunits of Microtubules

Answer 10

a/B tubulin heterodimer
(GTP bound)

Question 11

Describe the two ends of a microtubule

Answer 11

B-capped/+end) - fast growing
a-capped/-end) - slow growing

Question 12

Describe the Tubulin C-Terminal Tail in terms of:
(i) Function
(ii) Properties (e.g., Charge)

Answer 12

(i) Tubulin Tail = site of multiple PTMs
(ii) Flexible, and contains multiple Glutamate residues (E-hook), which give it a net negative charge

Question 13

(GTP-Binding, GTPase)

Compare the properties of the a/B tubulin monomers

(2 Points)

Answer 13

• a-tubulin - binds to GTP during folding, which is not hydrolysed (non-exchangeable)
• B-tubulin - GTPase activity, binds to GTP which is hydrolysed (exchangeable)

Question 14

How does Polymerisation impact the bound GTP?

Answer 14

a/B tubulin heterodimer is a poor ATPase, however polymerisation induces conformational change that induces GTPase activity of B-subunit

Question 15

What is the purpose of the GTP.Tubulin Cap

Answer 15

Allows microtubule growth by protecting against depolymerisation from +ve end

Question 16

(Catastrophe, Rescue)

What is meant by the Dynamic Instability of microtubules?

(3 Points)

Answer 16

• Catastrophe - if cap is lost (due to enzyme activity or slowed growth), the MT can quickly shrink
• Rescue - shrinking microtubules can re-enter growth if sufficient polymerisation occurs

MTs are said to exhibit dynamic instabilty as they can rapidly switch from shrinkage to growth in response to a variety of factors

Question 17

Why are Multistranded Filaments more suitable for the Cytoskeleton than Single-Stranded Filaments?

(2 Points)

Answer 17

• To achieve structures with similar mechanical strength/stability, the subunits in single stranded filament must be extremely tightly bound, therefore would have no dynamics
• Multistranded Filaments - both strong and dynamic

Question 18

Define Persistence Length

Answer 18

The length over which filament behaves as a rigid rod