Cytoskeleton Proteins

Question 1

What are the kinds of cytoskeleton filaments:

Answer 1

microtubules, actin filaments, intermediate filaments.

Question 2

describe Intermediate filaments -

Answer 2

no polarity, strong, ropelike, hard to break, coiled coils form tetramers, doesn’t bind triphosphate.

Question 3

What are the diseases linked to intermediate filaments?

Answer 3

Epidermolysis bullosa complex - failure to form filaments, can’t withstand stress resulting in cell rupture,
ALS - Accumulation of neurofilaments in the cell body and axons
Progeria - defects in nuclear lamins

Question 4

describe microtubule structure -

Answer 4

hard to break, stiff and difficult to bend, polar, includes alpha and beta tubulins that form protofilaments.

Question 5

How does nucleation of microtubules happen-

Answer 5

occurs at gamma tubulin ring complex in microtubule organizing center, complex associates with minus end.

Question 6

How is the centrosome related to microtubules?

Answer 6

It functions as the MTOC in animal centers, contains pair of centrioles, which ensure centrosome duplication during cell division.

Question 7

What affects microtubule stabilization?

Answer 7

MAP stabilizes them, resulting in less dynamic microtubules. Kinesin 13 (catastrophe factor) causes destabilization, resulting in more dynamic microtubules.

Question 8

What do capping proteins do?

Answer 8

stabilize microtubules by binding to plus end.

Question 9

What is dynamic instability? .

Answer 9

Change from growth to rapid shrinking caused by change in GTP binding. Rapid growth occurs with GTP cap and loss of cap causes catastrophe and shrinkage

Question 10

What do TIP Proteins do?

Answer 10

Modulate growth and shrinkage, control microtubule positioning.

Question 11

What are regulators of microtubule function?

Answer 11

kinesin 13 - enhances catastrophic disassembly at plus end,
katanin - severs microtubules
stathmin - binds subunits and prevents assembly
tips- remain associated with growing plus ends
can link them to other structures such as membranes, MAPs- stabilize microtubules.

Question 12

What helps formation of microtubule bundles?

Answer 12

MAP2 and tau

Question 13

Describe Actin Filaments:

Answer 13

two stranded helix, flexible and easy to bend, easy to break, polar with more dynamic plus end.

Question 14

How does actin filament nucleation occur?

Answer 14

It happens at the cell cortex and involves Actin related proteins that associate with minus end, involved in lamellipodia formation at leading edge for movement.

Web like networks of actin filaments formed.

Question 15

What does formin do?

Answer 15

Form dimer that recruits two actin monomers, associates with plus end.

Question 16

What does profilin do?

Answer 16

binds to formin whiskers, can increase rate of actin elongation.

Binds actin to prevent association with minus end, can only add to plus end.

Addition of actin to plus end results in conformation change
and release of profilin

Question 17

Describe treadmilling:

Answer 17

Plus end is more dynamic, minus end more likely to be shrinking, subunits at minus end are bound to GDP which causes instability, one end grows, one end shrinks.

Question 18

What is critical concentration and how does it relate to subunit addition/loss?

Answer 18

Concentration of available actin monomers. If the concentration of actin at one side is greater than Cc, subunits are added. If concentration at one side is lower than Cc, subunits are lost.

The minus end has a higher Cc than the plus end, so subunits usually lost from this side and added to plus end, resulting in treadmilling.

Question 19

Describe assembly/disassembly of subunits.

Answer 19

Occurs quickly as subunits are held noncovalently. They disassemble and assemble where needed due to easy diffusion of subunits.

Question 20

Distinguish fimbrin, alpha actinin, filamin, spectrin, and ERM

Answer 20

fimbrin and a-actinin - form parallel structures
filamin - forms web like structures,
spectrin and ERM help actin associate with membrane.

Question 21

What are stress fibers, cell cortex, and filopodium made of?

Answer 21

stress fiber - contractile bundle, cell cortex - gel like network, filopodium- tight parallel bundle

Question 22

Describe web like structures:

Answer 22

Important for lamellipodia formation, filamin links actin filaments, loss of filamin in tumor cells makes them less invasive.

Question 23

What is responsible for filament formation in actin and microtubules?

Answer 23

Actin: ARP 2/3, formin with profilin,
Microtubules: gamma tubulin ring complex (MTOC, centrosome)

Question 24

What severs filaments in actin and microtubules?

Answer 24

Actin: Gelsolin
Microtubules: Katanin

Question 25

What binds subunits to prevent assembly in actin and microtubules?

Answer 25

Actin: Thymosin
Microtubules: stathmin

Question 26

What regulates assembly/disassembly/stability in actin and microtubules?

Answer 26

Actin: Capping proteins, tropomodulin, tropomyosin, Microtubules: MAPs, TIPs

Question 27

What promotes disassembly in actin and microtubules?

Answer 27

Actin: Cofilin
Microtubules: catastrophe factors