Actin/Cytoplasmic Filaments and Cell Motility

Question 1

Microtubule functions

Answer 1

Transport

Organelle arrangement

Mitosis

Cilia & flagellar movement

Question 2

Microfilament functions

Answer 2

Muscle contraction

Cell adhesion & motility

Microvilli

Smallest but most common

Question 3

Functions of intermediate filaments

Answer 3

Mechanical integrity

Motility

Scaffolds

Question 4

Duchenne Muscular Dystrophy

Answer 4

• X-linked mutation –> complete absence of dystrophin –> plasma membrane of muscle cells tears during contraction
• Progressive
• Walking on toes; protruding abdomen; shoulder & arms held back; weakness and muscle wasting

Question 5

Dystrophin

Answer 5

Links actin to the extracellular matrix by bindin a,B dystroglycan on the plasma membrane, which binds laminin*, perlecan, and agrin in the ECM

Question 6

Becker’s muscular dystrophy

Answer 6

Lower expression levels of dystrophin; less severe w/later age of onset than DMD

Question 7

Why do people with DMD have enlarged calves?

Answer 7

Damaged muscle tissue –> fibroblasts migrate there to replace destroyed muscle with collagen and scar tissue

Question 8

How does ATP control the formation of actin filaments?

Answer 8

ATP-G-actin is added more quickly to the + end, and polymerization activates the intrinsic ATPase activity of actin –> hydrolysis and dissociation of ADP+Pi makes the actin less stable such that it falls off at the - end

Question 9

What binds to ATP-G-actin and sequesters it from polymerization?

Answer 9

Thymosin B4 and profilin

Question 10

The (+) end of F-actin as a ___er critical concentration for ATP-G-actin, which is why it’s added more efficiently at the (+) end

Answer 10

lower critical concentration

Question 11

Cofilin

Answer 11

Binds F-actin subunits with ADP and breaks it into shorter pieces –> more (-) ends for disassembly

Question 12

Profilin

Answer 12

Enhances exchange of ADP for ATP on free G-actins –> regenerates more ATP-G-actin but also sequesters it

Question 13

Thymosin B4

Answer 13

Binds ATP-G-actin to sequester it form polymerization until there’s too little

Maintains a steady state level of F-actin

Question 14

Tropomodulin

Answer 14

Binds at the (-) end to stabilize F actin

Question 15

CapZ

Answer 15

Binds at the (+) end to prevent further polymerization

Controlled by signal transduction molecules and other regulatory proteins

Question 16

Formin

Answer 16

Proteins that stimulate assembly of long, linear F-actin in stress fibers and contractile rings

Question 17

Fibrim

Answer 17

organizes filaments into microvilli

Question 18

Arp 2/3 & WASp

Answer 18

Stimulates formation of branched filaments in the leading edge of cells

Controlled by signal transduction processes

Question 19

Function of spectrin

Answer 19

Organizes microfilaments at the cell cortex

Question 20

How do CDC42 and WASP control actin assembly?

Answer 20

CDC42 is a small GTPase of the Rho family activated by growth factors; it activates WASp to nucleate actin filaments by Arp2/3 complex

Question 21

During cell migration, what 4 things are happening?

Answer 21

• Leading edge:
  
  • ​Extension of plasma membrane to form lamellipodium by increasing length of actin
  • Adhesion of lamellipodium
• Translocation: bulk of cell cytoplasm moves forward
• Back of cell: De-adhesion & endocytic recycling of adhesion points as stress fibers contract

Question 22

In the leading edge of a mgirating cell, extension is nucleated by ___ and controlled by __ and __, two signaling GTP-binding proteins activated by growth factor cascade

Answer 22

Nucleated by Arp2/3

Controlled by Cdc42 and Rac

Question 23

At the trailing edge, ___ is a GTP-binding protein that signals activation of formation and..

Answer 23

Rho

• Activates rho kinase, which activates myosin II to contract the back of the cell
• Blocks Rac

Question 24

a-actinin

Answer 24

Organizes the F-actin of the stress fibers during migration; also important in muscle contraction

Question 25

**Integrins**

Answer 25

Transmembrane proteins that bind to the cytoskeleton and the ECM to fix cells into their tissues, forming **focal adhesions** A nonmotile cell expresses integrins to keep it in place; also used to generate intercellular signals that lead to focal adhesions

Question 26

Lamellipodia vs Filopodia

Answer 26

**Lamellipodium:** projection of polymerized actin mesh on leading edge; whole structure propels the cell across a substrate ## Footnote **Filopodia**: ribs of actin within lamellipodia that spread beyond the lamellipodium frontier

Question 27

Hereditary sphereocytosis

Answer 27

RBCs are small and fragile because of a defect in **spectrin, protein 4.1**, or **ankyrin** --\> lack of cortical connections with plasma membrane --\> anemia, hepatomegaly, bilirubing allstones

Question 28

Epidermolysis bullosa

Answer 28

Mutations of **keratins 5** or **14** causes defects in the junction between dermis & epidermis

Question 29

Intermediate filaments are not dynamic and don't require ATP for polymeriazation; instead __ controls subunit exchange

Answer 29

Phosphorylatoin

Question 30

**ALS/Lou Gehrig's disease**

Answer 30

* Mutations affecting various **neurofilaments** --\> abnormal accumulations of neurofilaments (**spheroids** or Lewy body-like deposits) in motor neurons * 10% of ALS cases are familial (FALS), typically AD * 8 different mutations in the **profilin1 gene (PFN1)** are associated with FALS

Question 31

You can characterize cancers via IF's. A metastic tumor stians positive for **acidic and basic keratins.** From what cell type did this tumor most likely originate?

Answer 31

Epithelium

Question 32

Neurons have what type of IF's?

Answer 32

neurofilaments and lamins

Question 33

Muscle cells, glial cells, and mesenchymal cells have what IF's?

Answer 33

Desmin Vimentin GFAP

Question 34

What's the problem?

Answer 34

Duchenne Muscular Dystrophy Scar tissue (collagen+blood vessels) repalcing the muscle fibers --\> contraction of fibrous scar tissue around muscle cells causes muscle deformity and dysfunction