Question 1

How is membrane protrusion by blebbing different from that by lamellipodia?
A) Blebbing usually occurs on a rigid substratum such as glass, whereas lamellipodia form on pliable substrata.
B) Blebbing is mostly observed in vitro, whereas lamellipodia are observed both in vitro and in vivo.
C) Blebbing does not involve myosin II activity, but formation of lamellipodia does.
D) Blebbing requires loss of membrane interaction with actin filaments, whereas lamellipodia require a close interaction between the two.
E) All of the above.

Accepted Answer

Blebbing is mostly observed in vivo where the substratum is pliable. Both lamellipodia formation and blebbing require the activity of myosin II at some point. One of the key differences between the two is in their requirements for membrane interaction with cortical actin filaments.

Question 2

Which of the following is NOT an example of a crawling cell?
A) Macrophage
B) Osteoclast
C) Keratocyte
D) Neural crest cell
E) Sperm

Accepted Answer

Sperm cell motility is achieved through flagellum-mediated swimming.

Question 3

In lamellipodia, actin polymerization is nucleated by ...(1), while depolymerization is catalyzed by ...(2).
A) (1) formin; (2) gelsolin
B) (1) Arp 2/3 complex; (2) gelsolin
C) (1) formin; (2) cofilin
D) (1) Arp 2/3 complex; (2) cofilin
E) (1) formin; (2) capping protein

Accepted Answer

The Arp 2/3 complex nucleates a two-dimensional web of actin filaments in a lamellipodium. Behind the leading edge, cofilin disassembles older filaments which contain ADP-actin.

Question 4

Considering the diagram below, which summarizes two signaling pathways initiated by activated GTPases of the Rho family, what letter (A to D) corresponds to each of the following? Your answer would be a four-letter string composed of letters A to D only, e.g. ABCD.
  
( )	Rac-GTP
( )	Rho-GTP
( )	Lamellipodia formation
( )	Stress-fiber formation

Accepted Answer

Rac-GTP (B) promotes lamellipodium formation (D) by activating WASp family members, which leads to the nucleation of branched actin networks by the Arp 2/3 complex, as well as by activating filamin, a gel-forming actin cross-linker. Rho-GTP (A), on the other hand, activates formin to create parallel actin bundles, inhibits cofilin to stabilize actin filaments, and increases myosin motor activity, which all lead to formation of more stress fibers (C).

Question 5

In the following schematic drawing, in which direction (1 or 2) is the keratocyte migrating? What is the approximate length of the cell indicated by the bar on the left? 
A) Direction 1; about 20 µm
B) Direction 1; about 2 µm
C) Direction 2; about 20 µm
D) Direction 2; about 2 µm
E) Either direction; about 2 µm

Accepted Answer

The lamellipodium forms at the front of the cell toward where it is migrating. The cell is about 20 µm across and can move as fast as 30 µm/sec.

Question 6

These proteins are found in the budding yeast Saccharomyces cerevisiae; they form "neck filaments" between a mother cell and its growing bud, and help polarize protein distribution between the two. These proteins ...
A) can polymerize to form filaments and sheets.
B) bind GTP.
C) are also involved in contractile ring formation during cytokinesis in animal cells.
D) form filaments that are thought to be nonpolar.
E) All of the above.

Accepted Answer

They are septins.

Question 7

Consider an actin subunit that has just been incorporated into an actin filament at the leading edge of a lamellipodium. Before its ATP is hydrolyzed, how does its distance from the leading front edge of the plasma membrane change over time? How does its distance from the F-actin minus end change over time?
A) Decreases; decreases
B) Decreases; remains constant
C) Decreases; increases
D) Increases; decreases
E) Increases; remains constant

Accepted Answer

The actin filaments at the leading edge form a web, the whole of which undergoes treadmilling. An incorporated actin in this web remains more or less immobile relative to the substratum, but is constantly moving away from the polymerizing plus end (and the membrane) and getting closer to the depolymerizing minus end.

Question 8

In a cross section of a vertebrate axon, longitudinally organized cytoskeletal proteins can be seen as dots, as shown in the schematic drawing below. What type of filaments do these dots represent? 
A) Microtubules and microfilaments
B) Microtubules and vimentins
C) Microtubules and neurofilaments
D) Microtubules and keratins
E) Microtubules and septins

Accepted Answer

Both of these filament types are found in an axon and are critical for its structure and function.

Question 9

What are the typical consequences of mutations in keratins and their associated proteins?
A) Neurodegeneration as a result of interference with normal axonal transport.
B) Muscle development defects as a result of sarcomere disorganization.
C) Skeletal and cardiac abnormalities as a result of a weakened nuclear envelope.
D) Cornea disorders as a result of cell rupture from mechanical trauma.
E) Developmental defects as a result of abnormal signaling related to the primary cilia.

Accepted Answer

Cell rupture caused by mechanical trauma, and disorganization or clumping of the keratin filaments, is the hallmark of diseases (such as cornea diseases and skin blistering) caused by mutations in keratins and their associated proteins.

Question 10

In the following schematic drawing of a polarized neutrophil engaged in chemotaxis, in what region (a or b) does Rac activity dominate? What other member of the Rho family dominates at the other region? 
A) Region (a); Rho
B) Region (a); Cdc42
C) Region (b); Rho
D) Region (b); Cdc42

Accepted Answer

At the front of the cell (toward the source of the bacterial peptides), the Rac pathway dominates and limits the activity of the Rho GTPase to the other end of the cell.

Question 11

Indicate whether each of the following structures has actin organized mostly in a one- (1), two- (2), or three- (3) dimensional arrangement. Your answer would be a four-digit number composed of digits 1 to 3 only, e.g. 1333.
( )	Stress fibers
( )	Invadopodia
( )	Lamellipodia
( )	Filopodia

Accepted Answer

3
2
1
 Filopodia and stress fibers are linear structures, lamellipodia form flat, cross-linked networks, while invadopodia protrude across tissue barriers as footlike protrusions.

Question 12

If a certain isoform of myosin II is depleted from a cell, stress fibers are lost and focal adhesions disappear. If these cells are placed on a surface coated with an array of flexible pillars to measure traction forces, would you expect the traction to increase (I) or decrease (D) in these cells compared to wild-type cells? Write down I or D as your answer.

Accepted Answer

With the loss of the stress fibers and focal adhesions as a result of the depletion of the myosin isoform, the traction forces on the substratum are expected to decrease compared to normal cells.

Question 13

SUN and KASH proteins embedded in the nuclear envelope provide connections between the organization of the nucleus and the cytoplasm. Which of the following pairs of proteins DO NOT bind directly to each other in these connections?
A) Nuclear lamina and SUN proteins
B) Plectins and KASH proteins
C) KASH proteins and microfilaments
D) KASH proteins and SUN proteins
E) SUN proteins and cytoplasmic intermediate filaments.

Accepted Answer

SUN proteins in the inner nuclear membrane bind to KASH proteins in the outer nuclear membranes. Through this interaction, nuclear lamina and chromosomes (which bind to SUN proteins) are connected to actin filaments, microtubule motors, or plectins (which bind to KASH proteins) in the cytoplasm.

Question 14

Indicate whether each of the following descriptions matches Cdc42 (C), Rac (R), or Rho (H) from the Rho family of monomeric GTPases. Your answer would be a three-letter string composed of letters C, R, and H only, e.g. HRR.
( )	It activates formin.
( )	It inhibits myosin II activity.
( )	When constitutively active, it induces the formation of many prominent stress fibers.

Accepted Answer

The Rho family proteins regulate the actin cytoskeleton and cell polarization in different ways by acting on different but overlapping sets of target proteins.

Question 15

Keratins are intermediate filaments that ...
A) are localized inside the nucleus.
B) are composed of 50% type I and 50% type II keratin proteins.
C) are bundled in the epidermis to make cells more flexible.
D) anchor the intermediate filament network at the adherens junctions.
E) All of the above.

Accepted Answer

Keratins give cells mechanical strength, and provide tough coverings for animals. Every keratin filament is composed of an equal mix of type I (acidic) and type II (neutral/basic) keratins. These filaments are also involved in anchoring the cell cytoskeleton at desmosomes and hemidesmosomes.

Quiz 16: The Cytoskeleton

How is membrane protrusion by blebbing different from that by lamellipodia?

Which of the following is NOT an example of a crawling cell?

In lamellipodia, actin polymerization is nucleated by ...(1) , while depolymerization is catalyzed by ...(2) .

In the following schematic drawing, in which direction (1 or 2) is the keratocyte migrating? What is the approximate length of the cell indicated by the bar on the left?

These proteins are found in the budding yeast Saccharomyces cerevisiae; they form "neck filaments" between a mother cell and its growing bud, and help polarize protein distribution between the two. These proteins ...

In a cross section of a vertebrate axon, longitudinally organized cytoskeletal proteins can be seen as dots, as shown in the schematic drawing below. What type of filaments do these dots represent?

What are the typical consequences of mutations in keratins and their associated proteins?

In the following schematic drawing of a polarized neutrophil engaged in chemotaxis, in what region (a or b) does Rac activity dominate? What other member of the Rho family dominates at the other region?

Indicate whether each of the following structures has actin organized mostly in a one- (1), two- (2), or three- (3) dimensional arrangement. Your answer would be a four-digit number composed of digits 1 to 3 only, e.g. 1333. ( ) Stress fibers ( ) Invadopodia ( ) Lamellipodia ( ) Filopodia

SUN and KASH proteins embedded in the nuclear envelope provide connections between the organization of the nucleus and the cytoplasm. Which of the following pairs of proteins DO NOT bind directly to each other in these connections?

Keratins are intermediate filaments that ...