A4

Question 1

In the cell, the concentration of actin monomer is higher than the concentration required for purified actin monomers to polymerize in vitro. Thymosin is a protein that can bind actin monomers. If you were to add a drug that inhibits the ability of thymosin to bind actin monomers, what effect would this have on actin polymerization?

Answer 1

increase the rate of actin polymerization

Addition of a drug that keeps thymosin from binding actin monomer will increase the rate of actin polymerization in the cell. Thymosin normally binds actin monomers in the cell to sequester monomers until they are needed for actin polymerization. The reason actin
monomers do not spontaneously form filaments in the cell, despite the high concentration of actin monomer, is because the monomers are normally bound by actin monomer binding proteins (such as thymosin) and thereby prevented from adding onto the end of
an actin filament.

Question 2

Which of the following types of cells would you expect to contain a high density of intermediate filaments in their cytoplasm?
(There is more than one correct answer for this question. You need to select all the correct answers to score full marks for this question. You will get credit for each correct selection).

1.	 Smooth muscle in the digestive tract

2.	 Plant cell

3.	 Nerve cell in the spinal cord

4.	 Amoeba proteus (a free-living amoeba)

5.	 Skin epithelial cell

6.	 Sperm cell

7.	 Escherichia coli

Answer 2

1, 3, 5

Cells that migrate rapidly from one place to another, such as amoebae (A) and sperm cells (F), do not in general need intermediate filaments in their cytoplasm, since they do not develop or sustain large tensile forces. Plant cells (G) are pushed and pulled by the forces of wind and water, but they resist these forces by means of their rigid cell walls rather than by their cytoskeleton. Epithelial cells (B), smooth muscle cells (C), and the long axons of nerve cells (E) are all rich in cytoplasmic intermediate filaments, which prevent them from rupturing as they are stretched and compressed by the movements of their surrounding tissues.
All of the above eukaryotic cells possess at least intermediate filaments in their nuclear lamina. Bacteria, such as E. coli (D),have none whatsoever.

Question 3

The hydrolysis of GTP to GDP carried out by tubulin molecules:
1.
allows the behaviour of microtubules called dynamic instability

2.	 tips the balance in favour of microtubule assembly

3.	 occurs because the pool of free GDP has run out

4.	 provides the energy needed for tubulin to polymerize

Answer 3

1

Question 4

Match the type of intermediate filament with its appropriate location.
Lamins

Answer 4

The nucleus

Question 5

Match the type of intermediate filament with its appropriate location
neurofilaments

Answer 5

nerve cells

Question 6

Match the type of intermediate filament with its appropriate location.
cytokeratin

Answer 6

epithelia

Question 7

Match the type of intermediate filament with its appropriate location.
vimentins

Answer 7

fibroblasts in connective tissue

Question 8

Consider the mechanism by which actin and tubulin polymerize. Which statement below does not describe something similar about the polymerization mechanisms of actin and microtubules?

Depolymerization initiates at the plus ends of filaments

Although both filaments can grow from both ends, the growth rate is faster at the plus ends.

Nucleotide hydrolysis promotes depolymerization of filaments.

The free subunits (actin and tubulin) that incorporate into the filament have nucleoside triphosphate molecules bound.

Answer 8

Depolymerization initiates at the plus ends of filaments

Question 9

Intermediate filaments are made from elongated fibrous proteins that are assembled into a ropelike structure. The figure below shows the structure of an intermediate filament subunit. You are interested in how intermediate filaments are formed, and you create an intermediate filament subunit whose α-helical region is twice as long as that of a normal intermediate filament by duplicating the normal α-helical region while keeping a globular head at the N-terminus and a globular tail at the C-terminus; you call this subunit IFαd. If you were to assemble intermediate filaments using IFαd as the subunit, which of the following predictions describes the most likely outcome?

Filaments assembled using IFαd will interact with different cytoskeletal components.

Sixteen tetramers assembled from IFαd will be needed for a ropelike structure to form.

Filaments assembled using IFαd will form dimers that are twice as long as dimers assembled from normal intermediate filaments.

Dimers of IFαd will form by interactions with the N-terminal globular head and the C-terminal globular tail.

Answer 9

c

Because the α-helical region is twice as long, you would predict that a coiled-coil dimer made up of two IFαd subunits would be about twice as long as a dimer assembled from a normal intermediate filament subunit.

Question 10

A. have a monomer that binds ATP

B. include keratin and neurofilaments

C. are important for formation of the contractile ring during cytokinesis

D. their stability involves a GTP cap

E. are the major component of the mitotic spindle

F. can be connected through desmosomes

Answer 10

actin filaments
intermediate filaments
actin filaments
microtubules
microtubules
intermediate filaments