Microfilaments (Lecture 18)

Question 1

True or False: Cells differ in internal organization, shape, and motility while the cytoskeleton supports its needs.

Answer 1

True

Question 2

True or False: Distribution and organization of cytoskeleton filaments structurally polarizes different cells.

Answer 2

True

Question 3

In epithelial cells, core bundles of actin ______ support the apical microvilli.

Answer 3

Microfilaments

Question 4

What are the four parts that make up the cytoskeleton?

Answer 4

Microfilaments, microtubules, intermediate filaments, nucleus

Question 5

In the macrophage cytoskeleton, migrating cells polarized with leading edge oriented in the direction of locomotion.

Answer 5

True

Question 6

True or False: Cytoskeletons components contain at least 3 of the 4 parts that make up the cytoskeleton.

Answer 6

False, only need to contain one or more

Question 7

Order microtubules, microfilaments and intermediate filaments from shortest to longest.

Answer 7

Microfilament, Intermediate, microtubule

Question 8

______ is organized into functional bundles by actin-binding proteins and may serve as tracks for ATP-powered myosin motor proteins

Answer 8

Actin microfilaments

Question 9

______ extend through cell and provide organizational framework and may serve as tracks for ATP-powered kinesins and dyneins (motor proteins).

Answer 9

Tubulin microtubules

Question 10

______ provide structural support for nuclear membrane, barrier functions in skin, hair, and nails and are not used as tracks by motor proteins.

Answer 10

True

Question 11

True or False: Cells sense extracellular signals and interpret them to direct arrangement of the cytoskeleton.

Answer 11

True, Detected by cell-surface receptors that activate signal transduction pathways

Question 12

______ is a disease that can cause dysregulation or cytoskeletal component defects.

Answer 12

Metastatic cancer, allow cells to break away from the generating tissue.

Question 13

What are the 9 types of microfilament organizations.

Answer 13

Microvilli, cell cortex, Adherens belt, filopodia, lamellipodium/leading edge, stress fibers, phagocytosis, moving Endocytic vesicles, contractile ring.

Question 14

The building block of microfilaments is ______ and is molded into different structures by ______ proteins.

Answer 14

Actin, actin binding

Question 15

True or False: Microfilaments have reverse assembly.

Answer 15

True

Question 16

True or False: Actin is not an abundant intracellular protein in eukaryotic cells.

Answer 16

False

Question 17

True or False: Actins from amoebae and animals are 80% identical.

Answer 17

True

Question 18

How many actin genes do yeast and humans have?

Answer 18

2 and 6

Question 19

What are the 3 different groups of actin based on charge.

Answer 19

a-actin, b-actin, y-actin

Question 20

______ are enriched in cell cortex & leading edge of motile cells.

Answer 20

b-actin

Question 21

______ are found in some stress fibers.

Answer 21

y-actin

Question 22

______ are associated with contractile .

Answer 22

a-actin

Question 23

Each actin molecule contains an ______ ion complexed with either ______.

Answer 23

Mg2+, ATP or ADP

Question 24

True or False: Actin is a ATPase that hydrolyzes ATP to ADP and Pi.

Answer 24

True

Question 25

What are the two forms of actin?

Answer 25

Globular polymer, G-actin Filamentous polymer. F-actin F-actin is a linear chain of G-actin subunits

Question 26

______ actin is separated into two lobes by a deep cleft, denatures quickly without a bound nucleotide, and addition of cations Mg2+, K+, Na+ to G-actin induces filament formation.

Answer 26

G-actin

Question 27

______ actin is arranged in helical structures of one another and the ATP binding clefts are all oriented to the - end.

Answer 27

F-actin

Question 28

When does F actin repeat itself?

Answer 28

72nm or 14 actin subunits`

Question 29

What end of the F-actin filament is favoured for the addition of actin sub units and what side is favoured for subunit dissociation of ATP.

Answer 29

+, -

Question 30

True or False: Electron microscopy can’t detect the ATP-binding cleft when looking at microfilaments

Answer 30

True

Question 31

When myosin binds to actin, the arrow heads are pointing towards the ______ end and the barbed end is pointing towards the ______ end.

Answer 31

-, +

Question 32

True or False: Actin filaments are dynamic with polymerization and depolymerization.

Answer 32

True

Question 33

True or False: Actin-binding proteins contribute to the stability and disassembly of filaments.

Answer 33

True

Question 34

True or False: G-actin cannot be purified at high concentration without forming filaments if the buffer has ATP and low levels of cation.

Answer 34

False, it can

Question 35

What are the 3 phases of actin polymerization.

Answer 35

Nucleation, elongation, steady state

Question 36

The Lag period in which G-actin subunits combine into an oligomer of 2-3 subunits is known as the.

Answer 36

Nucleation phase

Question 37

True or False: In the elongation phase, actin is being added to both the + and - end.

Answer 37

True

Question 38

What is the critical concentration of actin polymerization?

Answer 38

Below which filaments cannot assemble and above which filament form

Question 39

What is the steady state of actin polymerization?

Answer 39

*   Concentration of monomeric actin remains at the critical concentration\ *   Rate of assembly and dissociation are equal *   Mixture of filaments and monomers

Question 40

True or False: Actin grows 10x faster at + end then negative end.

Answer 40

True

Question 41

True or False: Dissociation of ATP-G-actin subunits is favoured at the (-) end.

Answer 41

False, similar at both ends

Question 42

What is the Cc of subunits at the + end?

Answer 42

0.12 µM

Question 43

What is the Cc of subunits at the - end?

Answer 43

0.60 µM

Question 44

During actin treadmiling, at the steady state the + and - end adds and dissociated at the same rate.

Answer 44

False, + adds and - dissociates at the same rate.

Question 45

What is the actin treadmill powered by?

Answer 45

ATP hydrolysis

Question 46

What are the 3 types of actin binding proteins?

Answer 46

Cofilin, Profilin, Thymosin-B4

Question 47

______ binds G-actin on the side opposite of the ATP-binding cleft (+ end?) and keeps essentially all free G-actin in ATP-bound state

Answer 47

Profilin

Question 48

______ specifically binds ADP-F-actin and allows the filament to break into short pieces with more (-) ends that can be disassembled simultaneously.

Answer 48

Cofilin

Question 49

What two ATP binding proteins enhance treadmilling by 10x.

Answer 49

Profilin and Cofilin

Question 50

______ is a actin monomer sequestering protein that binds excess ATP-G-actin and will be released when ATP-G-actin becomes low

Answer 50

Thymosin-B4

Question 51

What is an example of Thymosin-B4

Answer 51

Platelets rich in actin and thymosin-β4; allows burst of actin assembly during blood clotting

Question 52

—______ helps nucleotide exchange so ATP-actin ready for (+) end —  ______ fragments ADPactin filament regions, enhancing depolymerization —  ______ provides buffered reservoir of ATP-G-actin for polymerization

Answer 52

Profilin Cofilin Thymosin-B4

Question 53

What is the function of capping proteins?

Answer 53

Prevent continual growth and disassembly in an uncontrolled manner

Question 54

What are the two classes of capping protein at each end of the filament.

Answer 54

CapZ + end Tropomodulin - end

Question 55

The capping protein ______ blocks assembly and ______ blocks disassembly.

Answer 55

CapZ Tropomodulin