Cytoskeleton II

Question 1

What are the basic building blocks of microfilaments?

Answer 1

• globular actin (G-actin) monomers assemble to make two-stranded, helical filaments (F-actin)
• most contain actin-binding proteins

Question 2

What are microfilaments critical for?

Answer 2

• cell shape
• movement
• polarity

Question 3

Are microfilaments polarized?

Answer 3

yes (due to orientation in the filament and molecular asymmetry of the subunits)

Question 4

What does phalloidin do?

Answer 4

• from the highly toxic death cap mushroom

- binds and stabilizes F-actin (causing a net increase in actin polymerization)

Question 5

Where does actin filament nucleation typically occur?

Answer 5

plasma membrane

Question 6

What catalyzes nucleation?

Answer 6

• actin-related protein complex (ARP or Arp2/3) - nucleates microfilament polymerization from the minus end, allowing rapid elongation at the plus end. Results in branched actin filaments
• Forming - leads to parallel actin bundle formation

Question 7

Actin plays a key role in ___________ of epithelial cells.

Answer 7

polarization

Question 8

What are some of actin’s key functions?

Answer 8

• polarizing epithelial cells
• anchoring proteins that are involved in tight junction (TJ) and adherens junction (AJ) formation
• apical microvilli formation
• forms the core of brush border microvilli

Question 9

What happens when there is a decreased association of adherens junction proteins (cadherins and catenins) with actin?

Answer 9

• leads to internalization of cadherins and loss of cell-cell adhesion
• prerequisite for epithelial-to-mesenchimal (EMT) transition and cancer formation

Question 10

How do microfilaments form brush border microvilli?

Answer 10

• tight microfilament bundle forms the core
• all actin plus-ends are anchored in the apical protein cap of the microvillus
• actin bundles are held together by cross-linking proteins villain and fimbrin

Question 11

In what disease is loss of microvilli observed?

Answer 11

microvilli inclusion disease

Question 12

To what protein family do actin binding motor proteins belong?

Answer 12

Myosin

Question 13

Describe the structure of myosin proteins (heavy chain).

Answer 13

• structurally related to kinesins
• head region (ATPase activity and actin binding sites)
• tail region (involved in binding to other molecules)

Question 14

Describe the structure of myosin II.

Answer 14

• hetero-oligomers involve two heavy chains and two light chains
• coiled tails bundle with other myosin molecules to form bipolar assemblies (several hundred myosin)

Question 15

What helps stabilize the plasma membrane and determine the shape and movement of the cell surface?

Answer 15

microfibril-rich layer underlying the plasma membrane

Question 16

What do flipodia and lamellipodia do?

Answer 16

can adhere or detach to/from a cell substratum via appropriate adhesion molecules

Question 17

What are stress fibers made from and what do they do?

Answer 17

• made from F-actin
• inserted into large adhesive patches (focal adhesions)
• viewed in cell cultures
• focal-adhesion-anchored cells exhibit low motility

Question 18

What large family are the signaling molecules for actin organization (and thus cell shape)?

Answer 18

Ras GTPase

Question 19

What does Rho do?

Answer 19

formation of stress fibers and focal adhesions

Question 20

What does Rac do?

Answer 20

formation of veils

Question 21

What does Cdc42 do?

Answer 21

protrusion of filopodia

Question 22

How do neural cells reach destinations in many parts of the body?

Answer 22

axons have what’s called a nerve growth cone that moves like an amoeboid cell (the nerve cell body typically remains stationary)

Question 23

What do Rho GTPases respond to in order to change the actin cytoskeleton of a cell?

Answer 23

• chemokinesis - random migratory activity

- chemotaxis - directed migration (can be positive or negative)

Question 24

What are the four steps an amoeboid cell goes through during locomotion?

Answer 24

• protrusion (of lamellipodia, fillopodia)
• attachment
• traction
• detachment

Question 25

What drives protrusion of fillopodia and lamellipodia?

Answer 25

• polymerization of actin meshwork at the leading edge (plus ends are facing forward)
• this is controlled by Rac and involves WASp (Wiskott-Aldrich syndrome protein) -> stimulates Arp2/3 complex

Question 26

What does actin-related protein complex (ARP or Arp2/3) do?

Answer 26

nucleates microfilament polymerization from the minus end, allowing rapid elongation at the plus end. Results in branched actin filaments

Question 27

What does Forming do?

Answer 27

leads to parallel actin bundle formation

Question 28

How is attachment and traction achieved by amoeboid cell protrusions?

Answer 28

• they form attachments with the substratum and pull the cell forward
• generated by myosin in conjunction with actin microfilaments

Question 29

Wiskott-Aldrich syndrome

Answer 29

• X linked immunodeficiency disease
• results from WASp mutations
• symptoms include thrombocytopenia (from defective lamellipodia/platelet formation) and recurrent infections (macrophages and neutrophil leukocytes are migration and chemotaxis deficient)

Question 30

Lissencephaly

Answer 30

• severe defect of brain development - smooth cortical surface
• loss of function of n-cofilin, actin filament depolymerizing factor -> lack of neuronal migration -> mental retardation

Question 31

How does actin play a role in cytokinesis (last stages of cell division)?

Answer 31

formation and contraction of actomyosin ring drives the formation of the cleavage furrow and separation of the daughter cells