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MSK Exam 2 > Lecture 6: Contractile Proteins > Flashcards

Lecture 6: Contractile Proteins Flashcards

1
Q

What assembles actin filaments into its two general types of structures?

A

Accessory proteins

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2
Q

What are the two general types of actin-based cell structures?

A
  1. Actin bundles

2. Actin networks

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3
Q

Actin bundles

A

Actin bundles are closely packed cross-linked actins that are parallel.

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4
Q

Actin networks

A

Actin networks are loosely cross-linked that are orthogonal. They form a 3-D meshwork that is gel-like.

Thus, it is more flexible.

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5
Q

Parallel actin bundles

A

Actin filaments in parallel actin bundles are tightly cross-linked by peptides (fimbrin) that has at least 2 actin-binding sites.

Fimbrin is a monomer.

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6
Q

Where are parallel actin bundles found?

A

Microvilli. They support the projections of the plasma membrane.

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7
Q

What determines how the actin filaments are associated with one another in a actin bundle?

A

Size and shape of the cross-linking protein.

They are usually small and rigid.

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8
Q

Polarity of actin filaments

A

Actin filaments have polarity: a + and - end.

Actin is added to + end and removed from - end.

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9
Q

Contractile actin bundles

A

In contractile actin bundles, the actin filaments are loosely bundled. This is a result of the crosslinking protein, alpha-actin.

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10
Q

What causes contractile actin bundles to be looser?

A

Alpha-actin binds as a dimer, causing the filaments to be further apart. This allows myosin to interact during contraction.

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11
Q

Ex. of a contractile actin bundle?

A

Contractile ring during mitosis

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12
Q

What holds actin filaments in a network together?

A

Filamin, a large actin binding protein that binds actin as a dimer.

Binding sides are located on opposite ends of the dimer, creating a 3-D network

Important for cells with a lot of mechanical stress, that need flexibility

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13
Q

Actin networks are important for what type of cells?

A

cells that undergo alot of mechanical stress and need flexibility.

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14
Q

Where are actin networks abundant?

A

Below the plasma membrane. They determine the
shape of the cell,
help with movement
stability and flexibility

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15
Q

RBC’s have which actin binding protein?

A

Spectrin- acting binding protein in RBC

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16
Q

What is the role of spectrin on the RBC’s?

A

The cortical skeleton of RBCs is made up of actin networks via spectrin.

This allows the membrane of the RBC to be flexible when traveling from high to low pressure.

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17
Q

How do spectrin-actin associate with membrane proteins?

A

Interactions with [ankyrin and protein 4.1]

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18
Q

Mutations in the cortical cytoskeleton proteins (spectrin or ankyrin 4.1) of RBC’s result in what?

A

Hereditary spherocytosis

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19
Q

Hereditary spherocytosis

A

a mutation in the cytoskeleton proteins (spectrin or ankyrin 4.1) that decreases the [flexibility and stability] of RBC’s. RBC’s become abnormally shaped.

Causes:

  1. Decrease number of RBCs (anemia)
  2. Jaundice
  3. Splenomegaly
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20
Q

3 types of actin projections

A
  1. Pseudopodia
  2. Lamellipodia
  3. Filopodia
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21
Q

Filopodia

A

Filopodia are thin projections of the plasma membrane that is supported by actin bundles. The movement of these projects are based on addition of actin at the + end and removal from the - end.

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22
Q

Lamellipodia

A

sheet like projects at the [leading edge] of the cell made by actin networks.

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23
Q

Pseudopodia

A

Actin networks responsible for phagocytosis.

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24
Q

How many types of myosin are there?

A

20 in eurkaryotes. They are a part of a superfamily of motor proteins.

25
Q

How does myosin move along actin fiaments?

A

ATP hydrolysis

26
Q

Describe the myosin found in skeletal muscle.

A

Myosin in skeletal muscle is called [Myosin II].

It is a bipolar filament with 2 heads at one end.

The tails of nearby myosin associate to form a shaft of filament.

27
Q

3 parts of myosin

A

Head
Neck
Tail

28
Q

Head of myosin

A

Has

  1. Actin binding sites
  2. ATP binding sites: has ATPase activity
29
Q

Neck of myosin

A

Flexible

Binds myosin light chain peptides

30
Q

Tail of myosin

A

Tails of myosin intertwine to allow the heads to stay close together.

Membranes and organelles can also bind

31
Q

Class 1 myosin

Function:

A
  1. Associates with the membrane

2. Endocytosis

32
Q

Class 2 myosin

Function:

A

Contraction

33
Q

Class 5 myosin

Function:

A

Transports organelles along actin cytoskeleton

34
Q

Neck of class 5 myosin

A

longer so it moves faster

35
Q

How does myosin move along actin.

A
  1. Without ATP, myosin is bound to actin.
  2. Binding of ATP to the ATP binding site on the myosin head breaks the myosin-actin bond and actin is released and myosin is activated.
  3. Myosin remains cocked until it binds to actin.
  4. When the activated myosin molecule is bound to actin, it goes through a conformational change called a “power stroke” due to the release of an Pi.
  5. Powerstroke straightens the myosin and moves the actin filament.
  6. Remains bound until another ATP comes in and switches the ADP.
36
Q

What is the velocity of myosin dependent on?

A

Length of the neck.

Thus, longer neck= increased rate of movement

37
Q

Arrange the following in order of velocity:

Class 1, Class 2 and Class 5

A

Class 5> Class 1> Class 2

38
Q

In skeletal muscle, what direction does myosin move?

A

In the direction of the + end of actin, shortening the sarcomere (from z-disk to z-disk)

39
Q

+ ends of actin is located where?

A

at the z-disk

40
Q

In smooth muscle, the myosin is arranged differently. What is this arrangement?

A

Myosin II is still used.

Without Ca2+, myosin II is folded and inactive

41
Q

How do we activate myosin in smooth muscle and non-muscle cells?

A

Increase Ca2+ will cause the myosin to unfold and become active.

42
Q

Excitation-Contraction Coupling in Smooth Muscle.

A
  1. Ca2+ concentration increases due to signaling cells.
  2. Binds to calmodulin
  3. Ca2+-calmodulin complex will active a protein kinase called the myosin light chain kinase (MLCK)
  4. Myosin unfolds and is now active
  5. It can now reach up and grab actin to cause contraction.
43
Q

In smooth muscle, how do myosin and actin interact?

A

Myosin and actin are loosely arranged. Myosin can slide along actin for long distances without encountering the sarcomere.

44
Q

Termination of contraction in smooth muscle requires what?

A

Myosin light chain phosphotase (dephosphorylates).

45
Q

What are non-muscle cell actin-myosin structures like?

A

They are similar to those in skeletal muscle fibers, except they’re much less stable and organized.

46
Q

What is an example of a non-muscle cell actin-myosin structure?

A

Contractile ring in cytokinesis

47
Q

Contractile ring

A

In cytokinesis, [f-actin and myosin II] will form a transient contractile ring.

Myosin moves along the actin to create a cleavage furrow.

48
Q

Myosin and actin structures are also involved in transport of vesicles.

A

When myosin V&I binds cargo (a vesicle), it will unfold and become active. It moves toward the + end.
-Myosin VI will move toward the minus end.

When cargo (a vesicle), it will fold and become inactive.

49
Q

What other types of myosin are involved in transport of vesicles?

A

Myosin VI can move toward the - part of actin.

Myosin I can move toward the + end.

50
Q

Myosin-actin structures in cell migration

A

(Myosin I) is involved in cellular migration by attaching to the plasma membrane via actin stress fibers (contractile bundles that interact w mysoin.

Actin is added to the plus ends of lamellipodia (leading edge) of cell. This helps the cell membrane migrate.

Actin networks help to stabilize the leading edge.

Focal adhesions are bound to the cell via integrin. Integrin is bound to contractile bundles. Contractile bundles contract to pull the focal adhesions off.

51
Q

What is Duchene Muscular Dystrophy

A

A mutation in the dystrophin gene that causes progressive muscle wasting. As a result, people are confined to a wheelchair by age 12. Most kids die of respiratory failure by age 22.

52
Q

Features of the dystrophin gene

A

427 kDA rod-like protein that connects the [cytoskeleton to the basal lamina]. Thus, it helps to stabilize the membrane.

It is the largest human gene known; 79 exons. Hundreds of mutations are found that are mostly large internal deletions. 1/2 are in frame and 1/2 are out of frame.

53
Q

Beckers MD (less severe form) mutations

A

IN-frame mutations that reduces the size of the protein and reduced function

54
Q

DMD mutations

A

Out of frame mutations that cause little to NO expression of the dystrophin.

55
Q

Can small deletions or point mutations lead to muscular dystrophy?

A

Yeth.

56
Q

What exactly does dystrophin do?

A

It helps to stabilize the sarcolemma by relieving the stress of [lateral and longitudinal transmission of contraction].

57
Q

Loss of dystrophin results in what?

A

Muscle cells burst and die.

58
Q

What are Stress fibers

A

contractile bundles that interact with myosin 1 to them to help w cell migration

59
Q

DYSTROPHIN gene connexts what to what

A

Cytoskeleton to basal lamina

MSK Exam 2 (23 decks)

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