32: Contractile Proteins

Question 1

Dystrophin function

Answer 1

Links muscle fiber cytoskeleton + ECM/basal lamina; acts as a shock absorber for contraction, stabilizes sarcolemma

Question 2

Dystrophin structure

Answer 2

Super big; 427 kDa (largest human gene, 79 Exons), has numerous redundant coils

Question 3

What are 2/3rds of all dystrophin mutations

Answer 3

Large internal deletions

Question 4

Out of frame vs in-frame dystrophin mutation

Answer 4

Out of frame -> frameshift -> DMD

In frame -> BMD

Question 5

Genetics of DMD

Answer 5

X-linked recessive, affecting 1 in 3,500 boys

Question 6

A few treatment options for DMD

Answer 6

Gene therapy, dystrophin replacement, drugs to prevent exon skipping

Question 7

G actin structure

Answer 7

375AA polypeptide divided into two lobes, with an ADP or ATP bound to the bottom of the cleft, connecting the lobes

Question 8

F actin

Answer 8

Tight right-handed helix formed by two strands of G actin polymerizing

Question 9

Why is polarity of actin important?

Answer 9

For determining which way myosin moves along it

Question 10

Three steps of actin filament assembly

Answer 10

1. Nucleation
2. Elongation phase
3. Steady state

Question 11

Nucleation of actin

Answer 11

3 G-actin monomers aggregate into an oligomer

Question 12

ATP hydrolysis in actin formation

Answer 12

ATP is bound to monomers -> hydrolyzed to ADP + Pi following filament assembly

Question 13

Where are actin filaments particularly abundant in the cell?

Answer 13

Beneath plasma membrane

Question 14

A requirement for actin binding proteins

Answer 14

Must have at least two actin-binding domains (ABDs)

Question 15

Actin bundles vs actin networks

Answer 15

Actin bundles: cross-linked into parallel arrays (filament polarity aligned)
Actin networks: loosely cross-linked into orthogonal arrays (polarity doesn’t align)

Question 16

Two types of actin bundles and the protein that holds them together

Answer 16

1. Parallel bundles: fimbrin

2. Contractile bundles: a-actinin

Question 17

Parallel vs contractile bundles of actin: packing

Answer 17

Parallel bundles: tight packing -> prevents myosin II from entering
Contractile bundles: loose packing, allowing myosin II to enter for contraction

Question 18

Parallel bundle function

Answer 18

Supports projections of plasma membrane, increases cell surface, structural stability, allows for additional receptors to be placed on plasma membrane

Question 19

Example of parallel bundles vs contractile bundles

Answer 19

Parallel: microvilli
Contractile: contractile ring in mitosis

Question 20

Actin network function

Answer 20

Forms 3D meshwork for flexible, gel-like properties

Question 21

Example of a protein that holds actin networks together

Answer 21

Filamen

Question 22

Spectrin

Answer 22

An actin binding protein in plasma membrane of RBCs that creates a cortical cytoskeleton

Question 23

Two other proteins that assist spectrin

Answer 23

Ankyrin, protein 4.1

Question 24

HS mutation

Answer 24

Spectrin, ankyrin, protein 4.1

Question 25

What happens in hereditary Spherocytosis?

Answer 25

RBCs are spherical and have impaired deformity, reduced stability, difficult to enter capillaries -> membranes break down and RBCs die -> anemia, jaundice, splenomegaly

Question 26

Three domains of myosin and their functions

Answer 26

1. Head: binds actin and ATP
2. Neck: binds myosin light chain
3. Tail: intertwine to bring heads closer, bind membranes/organelles

Question 27

Powerstroke mechanism and neck of myosin

Answer 27

Longer neck -> proportional to powerstroke mechanism movement

Question 28

What direction on actin do myosin I, II, and V move towards?

Answer 28

Plus end

Question 29

Myosin I, II, and V structures

Answer 29

I: 1 heavy chain + head + neck
II: 2 heavy chains + 2 heads + 2 necks
V: 2 heavy chains + 6 light chains per neck

Question 30

Only single-headed myosin

Answer 30

Myosin I

Question 31

Myosin I function

Answer 31

Some associate directly with membranes through tail-lipid interaction

Question 32

Only myosin class that can assemble into bipolar filaments through tail interactions

Answer 32

Myosin II

Question 33

Myosin V function

Answer 33

Tails associate with organelles/cargo -> carry it along actin filament tracks

Question 34

Four important functions of myosin-powered movements

Answer 34

1. Skeletal muscle contraction
2. Smooth muscle contraction
3. Non-muscle cell contraction
4. Cell migration

Question 35

Titin structure and function

Answer 35

Attaches Z disc -> middle f myosin filament; stabilizes myosin and prevents overstretching, brings thick filament back to baseline after contraction

Question 36

What do mutations in titin cause?

Answer 36

Cardiomyopathies

Question 37

Components of thin filaments

Answer 37

Actin, troponin, tropomyosin

Question 38

Nebulin

Answer 38

Long protein wrapping around actin, determines length of thin filament

Question 39

What two proteins stabilize ends of actin?

Answer 39

CapZ: stabilizes + end
Tropomodulin: stabilizes - end

Question 40

What constantly pumps Ca from Cytosol into SR in a resting muscle?

Answer 40

SERCA; SR Ca ATPase

Question 41

Smooth muscle contraction steps

Answer 41

1. Ca levels rise, Ca binds calmodulin
2. Calmodulin activates MLCK
3. MLCK phosphorylates myosin light chains -> high affinity for actin
4. Powerstroke

Question 42

What happens when Ca levels drop after smooth muscle contraction?

Answer 42

MLCK becomes inactive, MLCP dephosphorylates myosin light chains -> low affinity state for actin -> relaxation

Question 43

Non muscle cell actin-myosin structures

Answer 43

Can transiently form actin-myosin structures similar to skeletal muscle as needed, but are less stable and less organized

Question 44

Cytokinesis

Answer 44

Bundles of F-actin and myosin II form a contractile ring -> cleavage furrow

Question 45

What factor is expressed in fibroblasts for the localization of actin, causing cell migration?

Answer 45

GFP-actin