L3 Cytoskeleton I

Question 1

Cytoskeleton protein subunits

3

Answer 1

Intermediate filaments
Microtubules
Microfilaments

Question 2

Intermediate filaments are polymers of? What do the monomers look like?

Answer 2

Fibrous tetramers

All monomers are rope like proteins with head-rod-tail structure

Monomer to dimer
Two dimers form tetramer

Form in staggered array

Question 3

What are the intracellular functions of intermediate filaments and what’s their relative stability?

Answer 3

Provide tensile strength
Provide structure
Distribute mechanical stresses throughout the cell and tissues via desmosomes and hemidesmosomes

Most stable

Question 4

Do intermediate filaments have a NTP binding molecule or polarity?

Answer 4

No and no

Question 5

Microtubules are polymers of what? How do they go from subunit to polymer?

Answer 5

Tubulin heterodimer (alpha and beta tubulin)

Multiple tubulin heterodimers form a chain called protofilament (alpha and beta alternate)

13 Protofilaments form hollow cylinder (alpha to alpha, beta to beta laterally)

Question 6

Do microtubule subunits have polarity or a NTP binding molecule?

Answer 6

Heterodimer have polarity/ plus and minus ends

Alpha and beta each bind one GTP
Therefore each subunit binds to 2 guanine nucleotides

Question 7

What are the functions of microtubules and what is their relative stability?

Answer 7

Provide cell polarity and trains tracks for movement

Intermediate stability

Question 8

What happens to the GTP bound to the alpha tubulin?

Answer 8

It’s never hydrolyzed or removed

Question 9

What happens to the GTP bound to the beta tubulin?

Answer 9

Can be hydrolyzed to GDP

Form “T” and “D” forms

T form hydrolyzed in mitochon
D form exchanged in cytosol

Question 10

Microfilaments are polymers of what? How do they assemble?

Answer 10

Actin monomers (1 protein)

They line up end to end to form filaments

Fibers are composed of 2 parallel protofilaments that twist around each other in helical formation

Question 11

What is an actin networks main function and relative stability?

Answer 11

Provide cell shape, just under membrane

Least stable

Question 12

Are actin monomers polar? Do they bind to a NTP?

Answer 12

Yes they’re polar , plus and minus end

Bound to ATP= T form
Hydrolyzed to ADP in actin filament
Bound to ADP= D form
Exchanged for ATP in cytosol

Question 13

What happens during the lag phase?

Answer 13

Nucleation

Question 14

Explain nucleation

Answer 14

Initial process of cytoskeleton protein subunit assembly

Rate limiting step

Few subunits assemble spontaneously but they’re unstable due to limited number of stabilizing noncovalent bonds

accessory proteins help stabilize subunit assembly by increasing the number of noncovalent bonds

Question 15

What happens after enough subunits have joined?

Answer 15

Elongation proceeds rapidly

Question 16

What happens during the growth phase?

Answer 16

Elongation

Question 17

Explain growth phase /polymerization

Answer 17

Subunits added end to end rapidly

Rate of polymerization (Kon) is higher than rate of depolymerization (Koff)

Minus end of subunit to plus end of filament

Actin/tubulin bound to non hydrolyzed ATP/GTP (actin/ beta tubulin) have much higher affinity for one another- so they form polymers

Shortly after polymerization ATP/GTP is hydrolyzed

Question 18

Assume subunit carrying ATP/GTP or the T forms are where?

Answer 18

Free-floating in cytosol or incorporated into polymer

Question 19

Assume subunits carry ADP/GDP or D forms are where? And why?

Answer 19

On polymer because T is hydrolyzed once it’s part of polymer

Question 20

T forms have higher affinity for

Answer 20

Other T forms than D forms

Therefore polymerization occurs at plus end quickly and hydrolysis lags behind

Question 21

Critical concentration of T forms occurs at a ______ subunit concentration than critical concentration of D forms

Answer 21

Lower

Due to T forms higher affinity for other T forms

Question 22

What’s critical concentration?

Answer 22

Concentration of subunits above which the polymer elongates and below which it shrinks

Question 23

What’s treadmilling?

Answer 23

When the plus end of an actin filament polymerizes and the minus end depolymerizes

Question 24

Describe dynamic instability

Answer 24

When microtubules rapidly shrink or elongate

Allows for translocation, mobility, and train tracks

Question 25

What happens during a catastrophe?

Answer 25

Microtubule shrinkage

If GTP cap is lost (hydrolysis catches up with elongation) D form subunits will dissociate from the plus end

A lot of shrinkage and minimal polymerization

Question 26

What happens during a rescue?

Answer 26

Microtubule elongation

As conc of free T form subunit increases , T forms can be added to D form subunits at the plus end regaining faster elongation