Lecture 7 Learning Objectives

Question 1

a. List and differentiate the three types of cytoskeleton components

Answer 1

i. Microfilaments (actin) (7nm thick)
ii. Intermediate filaments (8-10 nm)
iii. Microtubules (25nm)

Question 2

characteristics of actin monomers and filaments

Answer 2

i. Actin filaments: highly conserved among eukaryotes
ii. Up to 7 micrometers in length, 7 nm thick
iii. Organized in bundles and 3D networks
iv. Bind to specific transmembrane proteins either directly or indirectly
v. Exist as monomers (G-actin) and long chains (F-actin)

Question 3

actin polymerization

Answer 3

i. Each monomer (G-actin) has a binding site for ATP.
ii. Monomer binds to three others forming Trimer – displays polarity
iii. ATP-actin associates with growing (plus or barbed) end
iv. ATP is hydrolyzed to ADP following polymerization

Question 4

Treadmilling

Answer 4

Intermediate concentrations of G-actin favor a dynamic equilibrium between minus end and the plus end
Results in zero net growth
High concentrations of G actin = growth

Question 5

Cytochalasins

Answer 5

drugs that affect polymerization
Bind to barbed ends
Block elongation
Can inhibit movements (cell division)

Question 6

Phalloidin

Answer 6

drugs that affect polymerization
Binds to actin filaments and prevents dissociation
Can label with fluorescence

Question 7

Spectrin

Answer 7

Actin-binding molecule
Found in RBCs
Binds cortical cytoskeleton to the plasma membrane

Question 8

Dystrophin

Answer 8

Actin-binding molecule

Binds cortical cytoskeleton to the plasma membrane

Question 9

Villin and Fimbrin

Answer 9

Actin-binding molecule

Cross-links in microvilli

Question 10

Calmodulin and Myosin I

Answer 10

Actin-binding molecule

Cross-links actin to plasma membrane in microvilli

Question 11

Alpha-actinin

Answer 11

Actin-binding molecule

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

Question 12

Filamin

Answer 12

Actin-binding molecule

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

Question 13

Actin-binding molecules that control treadmilling

Answer 13

Thymosin
Profilin
Gelsolin
Cofilin
Arp 2/3
Phalloidin
Latrunculins

Question 14

Thymosin

Answer 14

Actin-binding molecules that control treadmilling

Captures actin monomers: prevents monomers from being polymerized

Question 15

Profilin

Answer 15

Actin-binding molecules that control treadmilling
Binds to actin monomers and prevents monomers from being polymerized
Facilitates exchange of bound ADP for ATP which favors polymerization
Only ATP-actin monomers can be assembled into F-actin

Question 16

Gelsolin

Answer 16

Actin-binding molecules that control treadmilling
Destabilizes F-actin and caps actin filaments, preventing loss and addition of G-actin
In presence of Calcium ion, fragments actin filament and remains bound to plus end

Question 17

Cofilin

Answer 17

Actin-binding molecules that control treadmilling

Triggers depolymeriztion of ADP-bound actin at the minus end

Question 18

Arp2/3:

Answer 18

Actin-binding molecules that control treadmilling

Initiates growth of F-actin from sides of existing filament- causes branching

Question 19

Phalloidin

Answer 19

Actin-binding molecules that control treadmilling

Prevents depolymerization by binding to actin filaments

Question 20

Latrunculins

Answer 20

Actin-binding molecules that control treadmilling

Binds to G-actin and induces F-actin depolymerization

Question 21

basic structure of intermediate filaments

Answer 21

8-10nm thick – abundant in cells subject to mechanical stress
Provide tensile strength
Strengthen epithelial cells Head and tail domains impart specific function
Form a cytoplasmic network in most cells
Associate with other cytoskeletal elements to form a scaffolding that organizes the internal structure of the cell

Question 22

Formation of intermediate filaments

Answer 22

Two monomers form a dimer, two dimers align antiparallel to form tetramer.
Tetramers align end to end to form protofilament
Pairs of protofilaments associate laterally to form protofibril
Four protofibrils wind up to form rope – final product

Question 23

structure of a microtubule

Answer 23

Composed of tubulin dimers – alpha and beta dimers
Protofilaments are longitudinal rows of tubulin dimers
Microtubules consist of thirteen protofilaments arranged parallel to form a cylinder with a hollow core
Fast growing plus end and slow growing minus end

Question 24

functions of the cytoskeleton

Answer 24

Cell movement 
Support and strength for the cell 
Phagocytosis 
Mitotic spidle formation 
Cytokinesis 
Cell-to-cell and cell-to-extracellular matrix adherence 
Changes in cell shape