Lecture 6: Cytoskeleton

Question 1

Cytoskeleton is made up of these three specific protein structures

Answer 1

Microfilaments (actin)-7nm thick
Intermediate filaments- 8-10nm thick
Microtubules- 25nm in diameter

Question 2

Characteristics of Actin

Answer 2

Highly conserved among eukaryotes
7nm thick, up to 7um in length
Organized into bundles and 3D networks
Bind to specific transmembrane proteins directly or indirectly
Exist as monomers (G-actin) and long chains (F-actin)

Question 3

Why do actin filaments display polarity

Answer 3

Because all actin monomers are oriented in the same direction

Question 4

First step in actin polymerization

Answer 4

Nucleation: a trimer is formed, then additional actin can be added to either side

Question 5

The growing end is barbed or pointy

Answer 5

Barbed

Question 6

Treadmilling

Answer 6

Intermediate concentrations of G-actin favor a dynamic equilibrium between the +/- end. No net growth

Question 7

Low concentrations of G-actin favor

Answer 7

Depolymerization of actin filaments

Question 8

Higher concentrations of G-actin favor

Answer 8

Net addition of actin at both ends, therefore growing the actin filament

Question 9

Cytochalasins

Answer 9

Bind to barbed ends of actin, block elongation

Can inhibit movements (e.g., cell division)

Question 10

Phalloidin

Answer 10

Binds to actin filaments and prevents dissociation

Can be labeled with fluorescent dyes to allow visualization of actin filaments

Question 11

Diversity of function of actin filaments in a cell is determined by

Answer 11

Actin-binding proteins

Question 12

Spectrin

Answer 12

Found in RBCs

Binds cortical cytoskeleton to plasma membrane

Question 13

Dystrophin

Answer 13

Binds cortical cytoskeleton to plasma membrane

Question 14

Villin and Fimbrin

Answer 14

Cross-links actin in microvilli

Question 15

Calmodulin and Myosin I

Answer 15

Cross-links actin to plasma membrane in microvilli

Question 16

alpha-Actinin

Answer 16

Cross-links stress fibers and connects actin to protein-plasma membrane complexes

Question 17

Filamin

Answer 17

Cross-links actin at wide angels to form screen-like gels

Question 18

Thymosin

Answer 18

Captures/bind actin monomers, prevents them from being polymerized

Question 19

Profilin

Answer 19

Binds to actin monomers and prevents them from being polymerized
Facilitates exchange of bound ADP for ATP-which favor polymerization

Question 20

Gelsolin

Answer 20

Destabilizes F-actin and caps actin filaments, preventing loss and addition of G-actin
In presence of calcium ion, it fragments actin filaments and stays bound to +end

Question 21

Cofilin

Answer 21

Triggers depolymerization of ADP-bound actin at the minus end

Question 22

Arp2/3

Answer 22

Initiates polymerization of F-actin from sides of existing filament (causes branching)

Question 23

Latrunculins

Answer 23

Binds to G-actin and induces F-actin depolymerization

Question 24

How thick are intermediate filaments

Answer 24

8-10nm

Question 25

Intermediate filaments are abundant in cells subjected to

Answer 25

Mechanical stress

Question 26

Intermediate filaments strengthen epithelial cells as

Answer 26

Desmosomes and hemidesmosomes

Question 27

Intermediate filament central unit

Answer 27

Alpha-helical rod flanked by head and tail domains

Question 28

Head and tail domains of intermediate filaments determine

Answer 28

Specific functions

Question 29

Intermediate filament functions

Answer 29

Form a cytoplasmic network in most cells | Associate with other cytoskeletal elements to form a scaffolding that organizes the internal structure of a cell

Question 30

Dimer

Answer 30

Central rod of two polypeptides together | Rods are alined tail-tail, head-head

Question 31

Tetramer

Answer 31

Dimers associated in a staggered anti-parallel fashion

Question 32

Why do intermediate filaments not exhibit behaviors such as treadmilling

Answer 32

They are more stable than actin because they do not have distinct ends, due to the antiparallel association of dimers

Question 33

Protofilament

Answer 33

Tetramers assemble end to end

Question 34

Filament

Answer 34

Eight protofilaments wound together

Question 35

Most common type of intermediate filament

Answer 35

Keratin

Question 36

Microtubule thickness

Answer 36

25nm

Question 37

Microtubules are composed of what units

Answer 37

alpha and beta units

Question 38

Protofilaments are

Answer 38

longitudinal rows of tubulin dimers

Question 39

How many protofilaments arrange in what orientation for microtubules

Answer 39

13 parallel protofilaments

Question 40

____ dimers with ___ bound to the _____ associate with the growing end

Answer 40

Tubulin dimers with GTP bound to the beta unit

Question 41

Plus end grows more rapidly than minus end in presence of

Answer 41

low calcium ion concentration

Question 42

After polymerization, ___ is hydrolyzed to ___ and the tubulin is ____ stable

Answer 42

GTP is hydrolyzed to GTP and tubulin is less stable

Question 43

Dynamic instability

Answer 43

Individual microtubules undergo alternate phases of slow growth and rapid depolymerization

Question 44

alpha tubulin associates with the ___ end, beta tubulin associates with the ___ end

Answer 44

alpha tubulin=minus end | beta tubulin=plus end

Question 45

Colchicines

Answer 45

Inhibit microtubule polymerization

Question 46

Colcemid

Answer 46

inhibit microtubule polymerization

Question 47

Vincristine

Answer 47

inhibit microtubule polymerization

Question 48

Taxol

Answer 48

Stabilize microtubules

Question 49

Vinblastine

Answer 49

Inhibit microtubule polymerization

Question 50

Nocodazole

Answer 50

Inhibits microtubule polymerization

Question 51

Cytoskeleton functions (7)

Answer 51

``` Cell movement Support and strength for cell Phagocytosis Mitotic spindle formation Cytokenisis Cell-cell, cell-extracellular matrix adherence Changes in cell shape ```

Question 52

Kinesin

Answer 52

Anterograde transport protein

Question 53

Dynein

Answer 53

Retrograde transport protein

Question 54

Raft protein complex

Answer 54

May provide a mechanism for transport of multiple cargos

Question 55

Myosin I number of heads, tail binds to what, head binds to what, direction of head motion towards the

Answer 55

One head Tail binds to cell membrane Head binds to actin Head motion towards barbed end

Question 56

Myosin II number of heads, tail binds to what, head binds to what, direction of head motion towards the

Answer 56

Two heads Tail binds to Myosin II Head binds to Actin Head motion towards barbed end