Lecture 21 - Cytoskeleton: muscles

Question 1

Myosin II

Answer 1

• composed of 2 heavy chains and 4 light chains
• dimerization occurs when the two alpha helices of the heavy chains wrap around each other to form a coil
• each myosin head binds and hydrolyzes ATP, using that energy to walk toward the plus end

Question 2

Myosin in skeletal muscle

Answer 2

the tail-tail interactions from many myosin molecules form large, bipolar thick filaments that have several hundred myosin heads, oriented in opposite directions at the 2 ends

Question 3

Skeletal muscle

Answer 3

= muscle fibers
- big, multi-nucleated cells formed by the fusion of many muscle cell precursors
- sarcomere = contracting unit

Question 4

Sarcomere

Answer 4

formed from a miniature, precisely ordered array of parallel and partly overlapping thick and thin filaments

Question 5

Myofibril

Answer 5

a cylindrical structure that is often as long as the muscle itself
- consists of long repeated chains of sarcomeres

Question 6

Sliding filament theory

Answer 6

a muscle fiber contracts when myosin filaments pull actin filaments closer together and thus shorten sarcomeres with in a fiber
- actin filaments slide on myosin filaments
- A-band does not change
- I band disappears

Question 7

Thin and thick filaments: attachment

Answer 7

thin filaments are attached at their (+) ends to a Z disc at each end of the sarcomere
thick filaments are anchored at the M line

Question 8

What protein holds actin filaments together in a regularly spaced bundle?

Answer 8

a-actinin
- actin crosslinker => binds to actin filaments

Question 9

What is the Z dis built from?

Answer 9

CapZ => prevents polymerization and depolymerization

Question 10

Titin

Answer 10

acts as a molecular spring
- has a long series of immunoglobulin-like domains that can unfold as stress is applied to the protein
- keeps the thick filaments poised in the middle of the sarcomere (holds the sarcomere together)

Question 11

Nebulin

Answer 11

maintains the length and stability of the thin filament and consists almost entirely of a repeating 35-amino-acid actin-binding motif
- keeps actin stiff

Question 12

What initiates muscle contraction?

Answer 12

a sudden rise in cytosolic Ca2+ concentration

Question 13

Muscle contraction steps

Answer 13

1. a signal from a nerve triggers an action potential in the muscle cell plasma membrane
2. AP spreads into a series of transverse tubules (T tubules) that extend inward from the plasma membrane around each myofibril
3. Ca channels are activated in the t tubule membrane
4. these channels trigger the opening of Ca-release channels in the sarcoplasmic reticulum membrane
5. Ca floods into cytosol and initiates the contraction of myofibrils through the troponin complex and tropomyosin
   
   • tropomyosin = elongated protein that binds along the groove of the actin filament helix
   • troponin is a complex of 3 polypeptides
6. Ca binds to troponin C which causes troponin I to release its hold on actin
7. myosin binds to actin => muscle contraction

Question 14

What does troponin do in resting muscles?

Answer 14

the complex pulls tropomyosin into a position along the actin filament that interferes with the binding of myosin heads

Question 15

Why is the increase in Ca concentration transient?

Answer 15

because Ca is rapidly pumped back into the sarcoplasmic reticulum by and ATP-dependent Ca pump

Question 16

What is stretch activation?

Answer 16

an intrinsic length-sensing mechanism that allows muscles to function with an autonomous regulation (bypassing calcium)