Cytoskeleton

Question 1

What is a cytoskeleton? (4 functions)

Answer 1

protein structure that provides shape and helps maintain internal structure. Also involved in cell mobility, division, and intracellular trafficing

Question 2

Microtubule Properties

Answer 2

25nm in diameter, Function as scaffolds and assist with trafficking. All negative ends attach to centrosome.

Question 3

Microtuble Formation

Answer 3

Composed of Alpha and Beta tubulin dimers. Both bind ATP, alpha GTP covered after dimerization. Dimers bind together to make protofilaments which associate to make microtubules.
Polymeriation occurs from beta end where GTP is hydrolyzed. GTP cap stabilizes growing tubule.

Question 4

Intermediate Filament Properties

Answer 4

Types vary based on cell type. (Keratin in epithelia, Vimentin in connective tissue)
Provide mechanical stabilization to the cell and accessory proteins provide cross-linking

Question 5

Intermediate Filament Formation

Answer 5

Non-polarized, alpha helical, monomers that dimerize and coil. Dimers form staggered tetramers in anti-parallel fashion (C and N termini). Tetramers stack to from complex ropes.

Question 6

Microtubule stabilization

Answer 6

Growth stabilized by capping proteins

Question 7

Microtubule de-polymerization

Answer 7

Severing hexamers recognize C-terminal tails and utilize ATP for cleavage.

Question 8

Microtubule severing proteins (3)

Answer 8

Katanin, Spastin, Figetin

Spastin mutation associated with Hereditary Spastic Paraplegia

Question 9

Molecular Motors

Answer 9

Move cargo along microtubules. Form homo- or hetero-dimers.N terminus binds ATP and microtubule, A terminus binds adapter which binds cargo. ATP hydrolysis produces power stroke.

Question 10

Kinesin

Answer 10

Molecular motor moves in + direction away from centrosome toward PM

Question 11

Dynein

Answer 11

Molecular motor moves in - direction toward centrosome away from PM

Question 12

Microtubules in Mitosis (3 types)

Answer 12

3 microtubules develop in spindle following centrosome duplication.
Astral tubules form in all directions, stabilizing
Kinetocore tubules bind chromosomes
Overlap tubules attach to opposite overlap and help move chromosomes apart

Question 13

Taxol

Answer 13

Drug that prevents spindle assembly

Question 14

Hereditary Spastic Paraplegia

Answer 14

Spastin mutations reduce cleavage of excess microtubule branches in axons and can lead to cell death

Question 15

Keratin mutations

Answer 15

Disease occurs when all keratins in cell are mutated, most common in liver or kidney where only keratin 8 and 18 are expressed

Question 16

Intermediate filament diseases

Answer 16

Failure to cleave membrane binding Cas domains of laminins causes Progeria

Question 17

Actin Function

Answer 17

Involved in epithelial polarity, muscle contraction, cell motility, cell divison

Question 18

What do microtubules traffic?

Answer 18

mRNA, proteins, other microtubules (overlaps)

Question 19

Actin Formation

Answer 19

Helical filament of polar tetramers. Actin monomers (g-actin), do not form tetramers spontaneously (nucleation) and require either FH2 or Arp2/3 assistance.
Elongation and shrinking occur at either end but - end has exposed ATP that is needed for shrinking whereas + end ATP is covered.

Question 20

Regulation of Actin formation (3)

Answer 20

g-actin concentration, ADP-ATP exchange, capping/severing of filaments

Question 21

FH2

Answer 21

Helps with nucleation of elongating filaments. Mimics 2 g-actin for tetramer formation. ATP-independent, only needs activation by small GTPase

Question 22

Arp2/3

Answer 22

Helps with nucleation of branching filaments. Mimics 1 g-actin in pseudonucleation center. ATP-independent, only needs activation by small GTPase

Question 23

Epithelial Polarity

Answer 23

Actin forms tight and adherin junctions within cell that localizes appropriate proteins to appropriate membranes.
Also helps form microvili to improve absorption.

Question 24

Molecular Motion

Answer 24

Actin associates with myosin thick filaments where N-terminal head of myosin dimers perform power stroke with ATP hydrolysis

Question 25

Myosin 5

Answer 25

Irregular myosin, motor protein moving toward + end of actin

Question 26

Cell Movement

Answer 26

Leading edges of cells (lamellipodia) are covered in actin filaments produced by Arp2/3. Actin polymerization (via GTPase activation) elongates cell and causes binding to ECM through beta units associated with integrins. Contraction causes movement.

Question 27

Actomyosin ring in cell division

Answer 27

This contracts and pinches off cytoplasm to separate daughter cells. Localization of ring determined by Ect2 on astral filaments. At points where opposite astral filaments meet, Ect2 concentration is high enough to activate RhoGTP which activates ROCK which phosphorylates myosin and leads to ring contraction

Question 28

Asymmetric Cell Division

Answer 28

Erythrocytes cut out nucleus. Megakaryocyte divides but only splits off platelets. Ring canals form in spermatogonia. Sperm cells mature by releasing residual body. Epithelial cells divide only sideways to preserve monolayer