Unit 9

Question 1

2 functions of the cytoskeleton

Answer 1

1.) Provides a structural framework for the cell, serving as a scaffold that determines cell shape & the general organization of the cytoplasm
2.) Responsible for the movements of entire cells & for the internal transport of organelles & other structures (such as mitotic chromosomes) through the cytoplasm

Question 2

3 principal types of protein filaments

Answer 2

1.) Intermediate filaments
2.) Microtubules
3.) Actin filaments

Question 3

Intermediate filaments

Answer 3

• Diameter of ~10nm (intermediate)
• Not directly involved in cell movement
  Main function: Enable cells to withstand the mechanical stress that occurs when cells are stretched
  Appearance in the cell: IFs form networks throughout cytoplasm, surrounding nucleus & extending out to cell periphery, where anchored to cell membrane. Also indirectly connected to neighbouring cells through a cellular structure called desmosome

Question 4

Assembly of intermediate filaments

Answer 4

• Each monomer has an extended, central α-helical domain, & unstructured carboxy- & amino-terminal domains
• 2 monomers wrap around each other in parallel using their α-helical domains to form a coiled-coil dimer
• 2 dimers associate in a staggered & aniparallel fashion to form a tetramer. This means that a mature IF will not have polarity (one end of the IF resembles the other). This is in contrast to microtubules & microfilaments that do not have polarity
• 8 tetramers then laterally associate & are added to the growing IF. Unlike microfilaments & microtubules, there is no nucleation incolced & IFs build onto existing IFs. Thus, the IF network is not very dynamic
• Mature cytoplasmic IFs have a rope-like structure

Question 5

Nuclear lamina

Answer 5

• Intermediate filaments underlying the inner force of the nuclear envelope form the nuclear lamina, a fibrous network that
  1.) Supports the nuclear membrane
  2.) Provides attachment sites for the chromatin

Question 6

Microtubules

Answer 6

• Form rigid, hollow rods approx. 25nm in diameter. Continually undergo assembly & dissassembly
  Main functions:
  1.) Separation of chromosomes during mitosis
  2.) Intracellular transport of membrane-bound vesicles & organelles
  3.) Cell movement
• end: α subunits
  + end: β subunits

Question 7

What is the Microtubule organisation Center (MTOC) composed of?

Answer 7

1.) 2 centrioles - a unique arrangement of MT protofilaments at right angles to each other
2.) Pericentriolar material - an amorphous collection of several proteins
3.) γ-tubulin ring complexes - composed of a special form of tubulin called γ-tubulin & accessory proteins. Serves as the nucleation site for MT assembly

Question 8

Microtubule motor proteins

Answer 8

• Utilise energy derived from ATP hydrolysis to travel steadily along the microtubule in a single direction
  2 families:
  1.) Kinesins: Move towards the plus (+) end of microtubules
  2.) Dyeins - move towards minus (-) end of microtubules (towards centrosome)

Question 9

Movement of kinesin motor protein

Answer 9

1.) Trailing head has ADP bound, leading head has no nucleotide
2.) Binding of ATP by leading head induces a conformational change causing the former trailing head to move forward & become the leading head
3.) New leading head binds to MT ~16nm ahead of its previous side. Therefore the cargo moves in 8nm increments.
4.) Now leading head releases ADP. Trailing head hydrolyses ATP to ADP & P. The cycle is now ready to repeat

Question 10

Microfilaments

Answer 10

1.) ATP-actin monomers associate with the fast-growing plus (+) end
2.) The ATP bound to actin is hydrolysed shortly after polymerisation
3.) Hydrolysis of bound ATP to ADP in actin filaments reduces the strength of binding between monomers & decreases the stability of the polymer, leading to the dissociation of ADP-actin from the slow-growing minus (-) end
4.) Release of ADP-actin stimulates the exchange of bound ADP for ATP, resulting in the formation of ATP-actin monomers that can be re-polymerised into filmanets

Question 11

2 subfamilies of the myosin family of motor proteins

Answer 11

1.) Myosin I - found in all types of cells. Moves along actin filaments towards plus (+) end
2.) Myosin II most abundant in muscle
- Dimer w/ 2 globular heads & a tail that forms coiled-coil structure
- Associate through their coiled-coil tails forming myosin II filament