Lecture 15

Question 1

How do actin filaments subunits form F-actin?

Answer 1

Actin subunits assemble head to tail to form a tight, right-handed helix, forming a structure about 8nm wide called filamentous or F-actin.

Question 2

What is myosin II composed of?

Answer 2

Myosin II molecules are composed of two heavy chains and four light chains.

Question 3

When is myosin II dimerized?

Answer 3

Dimerization occurs when the two a-helices of the heavy chains wrap around each other to form a coiled coil.

Question 4

What do myosin heads bind and what do they do with it?

Answer 4

Each myosin head binds and hydrolyzes ATP, using the energy of ATP hydrolysis to walk toward the plus end of an actin filament.

Question 5

What occurs in muscles with tail-tail interactions?

Answer 5

In skeletal muscles, these tail-tail interactions form large, bipolar thick filaments that have several hundred myosin heads, oriented in opposite directions at the two ends.

Question 6

What is the bare zone?

Answer 6

The bare zone is the area without any myosin heads, where the heads begin going in different directions.
- These are coiled-coil interactions

Question 7

What cells are skeletal muscle?

Answer 7

Skeletal muscle cells are also known as muscle fibres.
- They are large and multinucleated cells

Question 8

How do skeletal muscle cells form?

Answer 8

Skeletal muscle cells form by the fusion of many muscle cell precursors.

Question 9

What are components of a muscle cell?

Answer 9

• multinucleated
• single membrane
• myofibrils (composed of actin and myosin`

Question 10

What is a sarcomere?

Answer 10

A sarcomere is the basic contractile unit of a muscle.

Question 11

Overview of skeletal muscle structure (muscle - sarcomere)?

Answer 11

• Muscle attaches to the tendons on either side, which then attached to the bones
• Myofibres are single muscle cells
• Myofibrils are actin and myosin, and contain….
• Sarcomeres are the basic contracile unit of the cell

Question 12

Z line?

Answer 12

The dark line on either side of the sarcomere

Question 13

I Band?

Answer 13

The light area in which the z line shows

Question 14

M line?

Answer 14

In the middle - no myosin heads present

Question 15

A-band?

Answer 15

Where myosin and actin are

Question 16

What are sarcomeres composed of?

Answer 16

Each sarcomere is formed from a miniature, precisely ordered array of parallel and partly overlapping thin (actin) and thick (myosin) filaments.

Question 17

What are myofibrils composed of?

Answer 17

A myofibril is a cylindrical structure 1-2 micrometers in diameter that is often as long as the muscle cell itself.
- Myofibrils consist of a long repeated chain of tiny contractile units (sarcomeres)

Question 18

What is the sliding filament theory?

Answer 18

The sliding filaments theory (1950):
- A muscle fibre contract when myosin filaments pull actin filaments closer together and thus shorted sarcomeres within a fibre.

Question 19

What pattern do we observe in skeletal muscles?

Answer 19

They have striated pattern:
- These striations disappear when the cell is contracted (shortened)

Question 20

What happens to the A and I band when the filaments slide?

Answer 20

Actin filaments slide on myosin filaments:
- A band (black) does not change
- I band (white) disappears

Question 21

What causes sarcomere shortening?

Answer 21

Sarcomere shortening is caused by the myosin filaments sliding past the actin thin filaments, with no change is the length of either type of filament.

Question 22

How are myosin and actin filaments packed?

Answer 22

Myosin and actin filaments are packed together with almost crystalline regularity.

Question 23

Where are thin filaments attached?

Answer 23

The thin filaments are composed of actin and associated proteins, and they are attached at their plus ends to the Z disc at the end of the sarcomere.

Question 24

Where are thick filaments attached?

Answer 24

The thick filaments are composed of myosin and they are anchored at the M line.

Question 25

What is the role of a-actinin?

Answer 25

a-actinin holds actin thin filaments together in a regularly spaced bundle: - They are an example of filament bundling proteins (also like fimbrin)

Question 26

Where are actin plus ends anchored?

Answer 26

The actin filament plus eds are anchored in the Z disc, which is built from CapZ. - CapZ will prevent further growing of the actin filament plus ends.

Question 27

What is the role of CapZ?

Answer 27

CapZ is a protein that effectively "caps" the growth of the actin filament plus ends

Question 28

What is the role of titin?

Answer 28

Titin acts as a molecular spring, with a long series of immunoglobulin-like domains that can unfold as stress is applied to the protein. - Titin keeps the thick filaments poised in the middle of the sarcomere.

Question 29

What is the role of Nebulin?

Answer 29

Nebulin maintains the length and the stability (keeps it straight) of the thin filament.

Question 30

What does Nebulin consist of?

Answer 30

Nebulin consists of almost entirely the same repeating 35 amino acid actin-binding motif

Question 31

What initiates muscle contraction?

Answer 31

A sudden rise in cytosolic Ca concentration initiates muscle contraction

Question 32

What does the signal from the nerve trigger?

Answer 32

The signal from the nerve triggers an action potential in the muscle cell plasma membrane, and this electrical excitation spreads swiftly into a series of transverse tubules (T tubules) - The T tubules extend inward from the plasma membrane around each myofibril

Question 33

What are T-tubules and what is their role?

Answer 33

T -tubules are invaginations of the membrane pulled into tubes. - they get in contact directly with the Ca containing ER - Ca release causes the myofibrils to contract.

Question 34

What is the impact of incoming action potential on Ca channels?

Answer 34

When the incoming action potential activates a Ca channel in the T-tubule membrane, it triggers the opening of a Ca-release channel in the closely associated sarcoplasmic reticulum membrane.

Question 35

Process of Ca channel opening?

Answer 35

- Ach is released binds to the receptors and opens sodium ion channels, leading to an action potential in the sarcolemma - Action potential travels along the t-tubules, - Calcium channel opens, causing muscle contraction => muscle contraction is from Ca signalling

Question 36

What does Ca flooding into the cytosol cause?

Answer 36

Ca flooding into the cytosol then initiates the contraction of myofibrils through the Troponin complex and Tropomyosin.

Question 37

What is tropomyosin?

Answer 37

Tropomyosin is an elongated protein that binds along the groove of the actin filament helix.

Question 38

What is troponin?

Answer 38

Troponin is a complex containing three polypeptides.

Question 39

What is the role of troponin?

Answer 39

Troponin, by working with tropomyosin and actin, binds actin and blocks myosin binding. - Actin is inacticated when Ca is bound.

Question 40

Role of troponin complex in resting muscle?

Answer 40

In a resting muscle, the troponin complex pulls the tropomyosin into a position along the actin filament that interfere with the binding of myosin heads.

Question 41

Troponin when Ca is raised?

Answer 41

When the level of Ca is raised, troponin C (which binds to Ca) causes troponin I to release its hold on actin. - Troponin moves tropomyosin when Ca is bound, pulling it away to revela a myosin binding site.

Question 42

Increase in Ca concentration is _______?

Answer 42

The increase in Ca concentration is transient, this is because the Ca is rapidly pumped back into the sarcoplasmic reticulum by an ATP-dependent Ca pump.

Question 43

What is stretch activation?

Answer 43

Stretch activation is an intrinsic length-sending mechanism that allows muscles to function with an autonomous regulation (bypassing calcium) - This is in our hearts - it also regulated the two different sets of muscles in the fly that controls its wings.