Cytoskeleton I

Question 1

Name three microtubule severing proteins.

Answer 1

• katanin
• spastin
• fidgetin

Question 2

Microtubules typically have one end attached to __________

Answer 2

a centrosome or perinuclear microtubule organizing center (MTOC)
- only one in mammalian cells

Question 3

What are the building blocks of microtubules?

Answer 3

• heterodimers of the protein tubulin (alpha and beta)
• GTP bound to both tubulins
• GTP bound to beta tubulin can be hydrolyzed and is exchangeable
• GTP bound to alpha tubulin cannot be hydrolyzed (trapped)

Question 4

What is the structure in regards to alpha and beta tubulins to form microtubules?

Answer 4

• alternating alpha and beta subunits create parallel protofilaments
• laterally, interactions are mostly alpha-alpha and beta-beta

Question 5

Why do microtubules have polarity?

Answer 5

Because one end has an alpha tubulin and the other has beta (due to the dimers)

Question 6

Why is the plus end the one that grows?

Answer 6

GTP bearing beta subunits favor polymerization

Question 7

Why does the minus-end tend to disassemble?

Answer 7

As dimers are incorporated more deeply into the microtubule, GTP is hydrolyzed to GDP, which weakens the tubulin interactions in the protofilament

Question 8

What happens if a microtubule loses its GTP-rich cap?

Answer 8

• rapid shrinkage from the plus-end until GTP containing dimers are added back
• called dynamic instability

Question 9

What do microtubule capping proteins do?

Answer 9

• bind to the ends of the microtubules

- increase stability

Question 10

What do microtubule severing proteins do?

Answer 10

• increase microtubule instability by exposing GDP rich parts of the microtubules
• e.g. spastin and katanin

Question 11

How does Paclitaxel work?

Answer 11

• binds to microtubules and stabilizes them

- causes tubule and tubulin aggregates

Question 12

Microtubules act as a __________ for organelle positioning.

Answer 12

scaffold (e.g. ER and Golgi are anchored to microtubules)

Question 13

How do microtubules assist with intracellular transport?

Answer 13

• acts as the tracks for microtubule motor proteins (head domain and cargo carrying domain)
• coordinated with ATP hydrolysis, motor goes through a mechanochemical cycle of binding, conformational change, release, conformational relaxation, re-binding

Question 14

Which direction do kinesins move cargo?

Answer 14

toward the plus-end of the microtubule

Question 15

Which direction do dyeins move cargo?

Answer 15

toward the minus-end of the microtubule

Question 16

What is the mitotic spindle constructed from?

Answer 16

Microtubules

Question 17

What are astral microtubules?

Answer 17

• contribute to the mitotic spindle

- radiate out form the centrosome

Question 18

What are kinetochore microtubules?

Answer 18

• contribute to the mitotic spindle

- attached to the kinetochore formed a the centromere of each duplicated chromosome

Question 19

What are overlap microtubules?

Answer 19

• contribute to the mitotic spindle

- interdigitate at the equator of the spindle

Question 20

Which direction does the plus end point in relation to the centrosome?

Answer 20

away

Question 21

What helps cause the centrosomes to grow apart during mitosis?

Answer 21

• plus-end directed, kinesin-like motor proteins bind to overlapping microtubules from opposite poles
• these motors and the elongation of the overlapping microtubules cause the spindle to grow

Question 22

What is flagella?

Answer 22

• have micro tubular core

- propel sperm

Question 23

What are cilia?

Answer 23

• have micro tubular core
• occur in large numbers on the apical surface of epithelial cells
• move fluids over the surfaces of cells

Question 24

What are intermediate filaments?

Answer 24

• fibrous structures of about 10nm
• prominent in cells exposed to mechanical stress
• provide intracellular mechanical support

Question 25

What are the two categories of intermediate filaments?

Answer 25

• cytoplasmic IFs

- nuclear lamins

Question 26

Are there cell specific cytoplasmic intermediate filament proteins?

Answer 26

Yes

• keratins
• vimentins
• neurofilament

Question 27

What do nuclear lamins do?

Answer 27

• filamentous proteins that form a stabilizing meshwork lining the inner membrane of nuclear envelope to provide anchorage for chromosomes and nuclear pores

Question 28

How are intermediate filaments formed?

Answer 28

• elongated molecules with extended central alpha helical domains that form a parallel coiled coil with another monomer
• form staggered tetramers in anti-parallel fashion
• not polarized
• tetramers assemble into protofilaments

Question 29

Where are intermediate filaments often anchored?

Answer 29

intercellular junctions

Question 30

How do keratin mutations result in epidermolysis bullosa simplex?

Answer 30

• interfere with filament assembly

- results in epidermis that is highly sensitive to mechanical stress and blisters easily

Question 31

How do mutations in the light chain of neurofilaments result in Charcot-Marie-Tooth syndrome?

Answer 31

may interfere with axonal transport of neurofilament subunits and cause peripheral neuropathy

Question 32

How do mutations in lamins result in various progeria syndromes?

Answer 32

can result in nuclear instability