Block IV: Skeletal muscle (Thick and Thin filaments)

Question 1

Where are thick filaments located, their measurements and composition?

Answer 1

1. Located within A-band of sarcomere
2. 1.5 - 1.6 microns long
3. Composed primarily of the protein myosin along with several
   accessory proteins

Question 2

identify

Answer 2

thick filaments

Question 3

What is the major protein in thick filament?

Answer 3

myosin

Question 4

WHat is the composition of myosin?

Answer 4

Myosin is composed of 2 heavy chains which form a coiled-coil tail domain (myosin rod) and two globular domains (myosin heads) at
one end of the rod

Question 5

What is associated which each head of the myosin?

Answer 5

Associated with each head are 2 light chains (the essential and regulatory light chains) involved in regulation (modulation) of myosin head binding to thin filament

Question 6

describe the characteristics of thick filament structure

Answer 6

1. bipolar structure - central bare zone & oppositely directed regions with myosin heads (crossbridges)
2. 30 nm diameter with a 15 nm backbone
3. heads on surface are near helically arranged (opposite polarity of heads)

Question 7

What does the M-band in thick filaments contain?

Answer 7

contains myomysium, M-protein, and obscurin

Question 8

What is the major accessory protein in thick filament structure?

Answer 8

Myosin Binding Protein C (MyBP-C) is the major accessory protein present as 7 - 9 bands across crossbridge region located in the Bare zone

Question 9

identify

Answer 9

thick filament

Question 10

identify

Answer 10

EM micrographs of isolated and negatively stained thick filaments
showing their structure and the ordered arrangement of the myosin heads on the surface of the thick filament (these are actually cardiac filaments with essentially same structure as skeletal thick filaments

Question 11

MyBP-C is the main accessory protein of?

Answer 11

skeletal and cardiac thick filaments

Question 12

How many domains does MyBP-C has, and where does it bind to?

Answer 12

It is a long linear molecule with either 10 or 11 domains. It binds to the myosin in the thick filament by domain 10 and is able to bind at its amino terminal to either the myosin head or the adjacent actin thin filament.

Question 13

what is the function of MyBP-C?

Answer 13

It Is thought to regulate or modulate the rate of contraction of both skeletal and cardiac muscle. In case of cardiac muscle is phosphorylated in response to adrenergic innervation and increases cardiac muscle crossbridge kinetics. Thus it
plays an important role in the modulation of contraction of both skeletal and cardiac muscle and does not simply act similar to the M-band proteins

Question 14

What are the characteristics of thin filaments?

Answer 14

~1 micron long

Double helical arrangement of actin + Ca+2 regulatory proteins -
tropomyosin & troponin.

Question 15

identify

Answer 15

EM of longitudinal section through sarcomere showing thin filaments.

Question 16

What is tropomyosin?

Answer 16

is a long rod-like protein that binds in a head-to-tail arrangement with other tropomyosin molecules and winds helically along
the thin filament as 2 strands on the double helical strands of actin

Question 17

Troponin binds to _________________ and consists of:

Answer 17

tropomyosin;
consists of 3 subunits:TnT (binding to tropomyosin), TnC(calcium binding), TnI (Inhibitory subunit that binds to F-actin as well as tropomyosin)

Question 18

What proteins are involved in the Ca+2 regulation of muscle contraction?

Answer 18

tropomyosin and troponin

Question 19

What are some other proteins in the sarcomere?

Answer 19

Titin
alpha-actinin
Tropomodulin
Nebulin
Dystrophin
Desmin

Question 20

Where is Titin located and its function?

Answer 20

Extends from the Z-line to the M-band. Binds along backbone of thick filament and has two elastic regions in Iband region.

Helps hold the thick filaments at center of sarcomere and helps stabilize sarcomere.

Question 21

Where is Alpha-actinin located?

Answer 21

Crosslinks actin filaments from adjacent sarcomeres at the Z-line.

Question 22

Where is Tropomodulin located and its function?

Answer 22

Caps the non – Z-line ends of the thin filaments thus stabilizing their length

Question 23

Where is Nebulin located and its function?

Answer 23

Wraps around thin filament and assists α-Actinin in maintaining binding of thin filament to Z-line

Question 24

Where is Dystrophin located and its function?

Answer 24

Attaches actin filaments in some places to transmembrane plasma
membrane proteins that in turn link to laminin in the CT, thus
providing attachment of the sarcomere to CT.

Question 25

What results in muscular dystrophy?

Answer 25

Loss of either dystrophin, the membrane complex proteins, or laminin gives muscular dystrophy

Question 26

Where is desmin located and its function?

Answer 26

attaches adjacent Zlines of sarcomeres in different myofibrils to each other thus stabilizing structure of muscle

Question 27

Which protein does Duchene muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) affect?

Answer 27

associated with mutations affecting the expression of the
dystrophin protein

Question 28

Which mutation is associated with Limb girdle muscular dystrophy (LGMD)?

Answer 28

associated with mutations on the short arm of the X chromosome for the 4 different sarcoglycan membrane proteins

Question 29

By what mutation is Congenital muscular dystrophy (CMD) caused?

Answer 29

caused by a mutation in the gene encoding the alpha-2 chain of muscular laminin

Question 30

identify

Answer 30

Normal Immunostaining for dystrophin

Question 31

identify

Answer 31

Immunostaining for dystrophin in Duchenne muscular dystrophy

Question 32

What is Desmin-related myopathy (DRM)?

Answer 32

A rare, inherited disease where due to a dominate mutation in desmin. The desmin molecules form aggregates rather than the desmin intermediate filaments. This results in disorganized myofilaments, myofibrils, and disorganized skeletal muscles

• Characterized by progressive weakness in muscles of legs, followed by weakness in trunk muscles and rest of body
• Because it also effects smooth muscle and cardiac muscle, respiratory insufficiency, heart failure, and misfunction in gastrointestinal functions may follow and lead to death
  often before age 50