The Cytoskeleton

Question 1

Why does a cell need its cytoskeleton?

Answer 1

• keep its shape and modify it in response to environmental cues.
  (complex network made of 3 different polymers)
  -microtubules - organelle positioning, intracellular transport
  -intermediate filaments- mechanical strength
  -actin filaments- cell shape, organelle shape and cell migration
  Provide for:
  1. shaping of the cell
  2. intracellular movement of organelles
  3. Cell movement

Question 2

Structure of the cytoskeleton?

Answer 2

• polymers are made from monomers
• monomers are very abundant
• not covalently linked

Accessory proteins regulate:

1. site and rate of filament formation (nucleation)
2. polymerization/ depolymerization
3. function

Question 3

Structure of microfilaments/ actin filaments

Answer 3

Helical polymers made of Actin

Flexible, organised into 2D networks and 3D gels

Cell shape

Organelle shape

Cell migration

Question 4

Structure of intermediate filaments

Answer 4

Heterogenous group of filamentous proteins

Rope-like structure

Give mechanical strength to the cell

Question 5

Structure of microtubules?

Answer 5

Hollow tubes made of tubulin

Rigid, long straight

Organelle positioning

Intracellular transport

Cell movement

Question 6

Microfilaments

Answer 6

Twisted chain of units (monomers) of the protein actin (g-actin). This chain constitutes the filamentous form (f-actin)

Thinnest class of the cytoskeleton filaments (7nm)

Presents structural polarity

Associated with a large number of actin binding proteins (ABP)

-variety of organisations and function

There are 3 isoforms of G-actin with different isoelectric point:

• alpha actin is found mainly in muscle cells
• beta actin and gamma actin is found in non-muscle cells

Question 7

Actin polymerisation

Answer 7

Actin filaments (F-actin) can grow by addition of actin monomers (G-actin) at either end.

ABP proteins binding to monomers

ABP proteins binding to filaments

Actin bundling proteins- keep F-actin in parallel bundles (as in the microvilli observed in epithelial cells)

Cross linking proteins- maintain F-actin in a gel-like meshwork (as seen in the cell cortex under the plasma membrane)

Question 8

What is the length of the filament determined by:

Answer 8

1. conc. of G- actin.

2. Presence of Actin Binding Proteins

Question 9

What are G actin levels controlled by?

Answer 9

1. Profillin: facilitates actin polymerization

2. Thymosin Beta4: prevents the addition of actin monomers to F-actin.

Question 10

How else is the cell able to control the length of the filaments?

Answer 10

F actin severing proteins- break F actin into smaller filaments

Motor proteins (myosin)- transport of vesicles and/or organelles through actin filaments.

Question 11

Functions of the actin filaments in skeletal muscles

Answer 11

Arranged in a para crystalline array integrated with different ABPs

Interaction with myosin motors allow muscle contraction

Question 12

Functions of the actin filaments in non muscle cells?

Answer 12

Cell cortex: form a thin sheath beneath the plasma membrane

Associated with myosin form a purse string ring result in cleavage of mitotic cells.