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Biochemistry Muscoskeletal > Cytoskeleton > Flashcards

Cytoskeleton Flashcards

1
Q

General functions of the cytoskeleton

A
  1. Maintain structural integrity
  2. Cell movement
  3. Causes changes in cell shape
  4. Contraction of muscles
  5. Moves organelles, vesicles, and protein to distal parts of the cell
  6. Provides binding sites for RNA and proteins
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2
Q

3 filamentous components of the cytoskeleton

A

Microfilaments

Microtubules

Intermediate filaments

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3
Q

Major protein component of each of the 3 cytoskeleton components?

A

Microfilaments = actin

Microtubules = alpha and beta tubulin

Intermediate filaments = varies according to the filament

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4
Q

General functions of each

A

Microfilaments = cell adhesion, cell movement, muscle contraction

Microtubules = move vesicles, granules, organelles, and chromosomes

Intermediate filaments = mechanical integrity of the cell, motility, scaffolds for signaling molecules

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5
Q

How does ATP control the formation of actin filaments

A

ATP-G-actin is added at a faster rate to the + end of the growing filament

Polymerization activates the intrinsic ATPase activity of the actin molecule

Once the ATP has been —> ADP + Pi…the association of the actin in the filament is less stable…and the actin monomer will tend to fall off the - end

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6
Q

Thymosin beta4 and profilin

A

Actin associated proteins that bind to the ATP-G-actin and sequester it from the polymerization process

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7
Q

Role of cofilin and profilin in the formation of actin filaments

A

Profilin:

Enhances exchange of ADP for ATP on G-actin and but also sequesters actin from the pool of polymerizable actin monomers…

Cofilin:

Bind to F-actin…whose subunit contain ADP…when it binds, it breaks the actin filament into shorter pieces…this generates more - ends and therefore enhances the disassembly of the filament

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8
Q

Function of CapZ and tropomodulin

A

CapZ:

Binds to the + end of the filaments and prevents the addition of new G-actin monomers

Tropomodulin:

Binds at the - end and stabilizes the filaments

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9
Q

Family of proteins that promote formation of long, unbranched filaments

A

Formin

Found in stress fibers and the contractile ring

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10
Q

What actin associated protein stimulate formation of branched filaments

A

WASp and the Arp2/3 complex

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11
Q

Function of fibrim

A

Organizes filaments in MICROVILLI

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12
Q

Function of spectrin

A

Organizes filaments at the CELL CORTEX

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13
Q

Role of CDC42 and WASp in actin assembly

A

CDC42:

Small GTPase of the Rho family that can be activated by growth factors…once activated it can activate Wiskott-Aldrich Syndrome protein (WASp)

Which leads to the nucleation of actin filaments by Arp2/3 complex

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14
Q

Difference between lamellipodia and filopodia

A
  1. Is a projection of polymerized actin projection on the mobile edge of the cell…contains a quasi-2-dimensional actin mesh…

The whole structure propels the cell across a substrate

Within the lamellipodia are ribs of actin called microspikes…which when they sperad beyond the lamellipodium are called filapodia

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15
Q

Hereditary spherocytosis

A

RBCs are small and fragile because of a defect in spectrin, protein 4.1, or ankyrin

Patients usually have anemia and enlarged spleens because of the enhanced breakdown of RBCs

Gallstones composed of bilirubin are also common

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16
Q

Epidermolysis bullosa

A

Mutations of keratins 5 or 14

Can cause defects in the junction between the epidermis and dermis

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17
Q

What does G-actin stand for?

A

globular-actin … has ATPase activity

One actin molecule not attached to anything

Every G-actin monomer has a binding site for ATP and Mg2+…which are necessary for actin polymerization (these sites are located in the ‘dimple’ of the molecule)

Molecule is considered ‘polar’ since this dimple is only on one side

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18
Q

What does F-actin stand for?

Key info about it?

A

Filamentous actin

Many monomers connected together (unlike G-actin)

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19
Q

Which side are actin monomers added to of the fiber?

A

(+) side

Aka…’dimple first’

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20
Q

Side that actin monomers are removed from

A

(-) side

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21
Q

Describe the ‘treadmilling process’

A
  1. G-actin binds ATP (ATP-G-actin)
    —> this makes it have a high affinity for the (+) end of the microfilament —> therefore add to the growing microfilament
  2. Once G-actin binds to the m.f. —> only a matter of time before it hydrolyzes it back to ADP+Pi (this action is delayed like a timer)
  3. Once this process happens —> ADP-G-actin is less stable and will dissociate from the (-) end of the filament
  4. ADP-G-actin remains in solution until its ADP is exchanged for another ATP
22
Q

Mechanism of Thymosin-Beta4

A
  1. Binds to free ATP-G-actin —> sequestering it
  2. This creates a RESERVE of ATP-G-actin
  3. When the free [ATP-g-actin] is low —> thymosin-beta4 releases some for polymerization purposes
23
Q

Mechanism of profilin

A
  1. Enhances the exchange of ADP —> ATP on G-actin…thus increasing the polymerization of the microfilament
24
Q

Mechanism of cofilin

A
  1. Binds to F-actin whose subunits have already hydrolyzed ATP to ADP
  2. Then breaks the filament into small pieces
  3. This creates more (-) ends than there were before —> enhancing the DISASSEMBLY of the filament
25
Q

Mechanism of CapZ

A

Binds to the (+) end

Prevents the addition of new monomers

26
Q

Tropomodulin mechanism

A

Binds to the (-) end

Prevents the depolymerization thus stabilizing the filament

27
Q

Why is myofibril in a muscle cell so stable?

A

The surrounding proteins stabilize the actin filaments

Specifically the CapZ protein in the Z-band…and the tropomodulin protein near the other end of the actin filament (beginning of H-band)

28
Q

The three sub-cycles within the actin polymerization cycle?

A
  1. The cofilin cycle —> disassembly
  2. The profilin cycle —> the exchange of ADP for ATP
  3. The thymosin-beta4 cycle —> releases ATP-g-actin from storage to add to the (+) end of the filament
29
Q

Role of nucleating proteins

A

Directly help in putting the actin together in a certain way

They are highly regulated by signaling transduction processes within the cell

Ex: Formin and Arp2/3 complex

30
Q

Formin

A

Promote assembly of a STRAIGHT, UNBRANCHED FIBER

Important for the formation of stress fibers and the contractile ring in cytokinesis

31
Q

Arp2/3 complex

A

A complex that forms filament BRANCHES

Important in lamellipodia —> little extensions in the leading edge of a migrating cell

32
Q

General role of cross-linking proteins

A

Take what filaments are already made and available and hold them in certain patterns and positions

DO NOT assemble, degrade or maintain filaments

Ex: fimbrin, alpha-actinin, spectrin

33
Q

Fimbrin

A

Organizes filaments in microvilli

34
Q

Alpha-actinin

A

Organizes filaments into stress fibers that help the cell contract

35
Q

Spectrin

A

Organizes filaments at the cell cortex

36
Q

Hereditary Spherocytosis

A

NORMALLY:

Cortical cytoskeleton consists of many proteins, each help keep the cortical cytoskeleton (remember actin) intact and associated with the membrane…helps keep the cell in the desired shape

Defect in spectrin, protein 4.1, OR ankyrin
—> failure of the cortical skeleton to retain shape

This effect is particularly noticeable in RBCs —> the body is constantly breaking down the faulty RBCs

Symptoms:

RBCs are small and fragile
Anemia (less functional blood)
Enlarged spleen (which recycles old RBCs)
Gallstones composed of bilirubin (byproduct of heme breakdown)
37
Q

Dystrophin

A

Connects actin (in the cytosol) to alpha,beta-dystroglycan (plasma membrane)

38
Q

Alpha,beta-dystroglycan

A

Transverse the plasma membrane to connect dystrophin to the ECM

On the ECM side —> has a O-andN-linked carbohydrates…which bind primarily to LAMININ is addtion to other basal lamina proteoglycans such as perlecan and agrin

These connections are thought to stabilize the muscle cell during contraction

39
Q

During cell migration…which nucleating protein is at the leading edge? Trailing edge?

A

Leading = Arp2/3 complex

Trailing = formin

40
Q

The 4 steps of cell migration

A
  1. Extension
  2. Adhesion
  3. Translocation
  4. De-adhesion and endocytic recycling

(All happening simultaneously)

41
Q

Proteins involved in the extension of the lamellipodium?

A
  1. Arp 2/3
  2. Cdc42 (G-protein)
  3. Rac (G-protein)
42
Q

Describe the formation of the focal adhesions

During the adhesion step of cell migration

A

Focal adhesions = the places where the cell attaches to the ECM it is migrating through

Made from integrins and cytoskeletal associated proteins as a results of cellular signaling

43
Q

Translocation step in migration

A

Bulk of the cytoplasm flows forward due to contractions at the rear of the cell

Formin

Myosin

Rho (G-protein) activates myosin and formin —> prevents Rac from operating, thus creating the necessary cell polarity

(Rac is involved in the extension of the cell)

44
Q

Detachment of the cell rear during migration

A

Endocytosis/recycling of the membrane proteins involved in cell adhesion

45
Q

Intermediate filaments (general)

A

Heterogeneous group of polymers

Form a network extending from the nucleus to the plasma mem. In most cells

Purpose = mechanical integrity, motility, scaffolds for signaling molecules

Know these: laminin, keratins, vimentin, desmin

They are dynamic - exchange subunits…regulated by phosphorylation (not ATP or GTP)

46
Q

Tumor containing keratin =

A

Epithelium

47
Q

Tumor containing GFAP =

A

Glial cell

48
Q

Acidic keratins and basic keratins

Location and function

A

Epithelial cells

Tissue strength and integrity

49
Q

Desmin, GFAP, vimentin

A

Muscle, glial cells, mesenchymal cells

Fxn: sacromere organization, integrity

50
Q

Neurofilaments

NFL, NFM, NFH

A

Neurons

Axon organization

51
Q

Lamins

A

Nucleus

Nuclear structure and org.

Biochemistry Muscoskeletal (8 decks)

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