WEEK 4

Question 1

Cytoskeleton=

Answer 1

• It is a filamentous structure which is found through out the cytoplasm and also in the nucleus
• It is formed of protein monomers which assemble into repeat structures – rather like bricks assembling into a wall
• It is dynamic – assembling and disassembling to suit the cells needs – but can persist to form a stable supporting structure

Question 2

What 3 distinct types of filament is the cytoskeleton composed of?

Answer 2

1. actin filaments
2. microtubules
3. intermediate filament

Question 3

Actin filaments:
- made of?
- how are they dispersed?
- role?

Answer 3

• Made of the globular protein actin which assembles into two stranded helical polymers which then:

– line up to form bundles

– or form 2D networks

– or form 3D gels

• They are dispersed through the cell, but are concentrated beneath the cortex.
• Roles in cell shape and motility

Question 4

Microtubules=
- made of?
- roles?

Answer 4

• Made of the globular protein tubulin (a and b subunits).
• These dimerize and then form hollow tubules.
• Are more rigid than actin filaments – long and straight.
• Typically one end is attached to a microtubule. organising centre (MTOC), while the other end grows and shrinks.
• Roles in positioning organelles, intracellular transport and mitosis.

Question 5

Intermediate filaments=
- made of?
- roles?
- structure?

Answer 5

• Made of various intermediate filament proteins which are themselves filamentous.
• Extended a helical regions wind together into dimers, which then associate into tetramers (soluble subunit) that wind together to form rope-like fibres.
• Roles in mechanical support of cell structures

Question 6

CYTOSKELETON ASSEMBLY=

Answer 6

• Monomers form end-to-end and side-to-side interactions Relative strengths determine mechanical properties
• Rate of monomer addition relative to monomer concentration Rate of disassembly is constant i.e. net growth depends on monomer availability
• At critical concentration of monomers: addition & removal at the same rate Cytoskeletal elements at EQUILIBRIUM PHASE: no net growth but still gradually turn over
• Auxiliary proteins can affect these processes – allowing the cell control
• Interactions are non-covalent rapid assembly/disassembly; no bonds to break compare to DNA, RNA and peptides!

Question 7

Dynamic control of filaments

Answer 7

Nucleation

Assembly

Disassembly

Question 8

Nucleation =

Answer 8

auxiliary proteins may act as an initiator for a new filament.

Question 9

Assembly =

Answer 9

proteins which bind monomers change their relative concentration, affecting addition rates.

Question 10

Disassembly =

Answer 10

proteins can bind and either stabilise or destabilise filaments.

Question 11

Motor Proteins:

Answer 11

Move organelles along filaments and filaments against each other