Contractile Proteins

Question 1

The Cytoskeleton

Answer 1

Define the stability, structure, shape, and is very dynamic

Structural, spatial and mechanical functions of cells depend on the cytoskeleton, a remarkable system of filaments scaffolding the cell

Question 2

Actin

Answer 2

major compotent of the cytoskeleton

each subunit is called a G actin (globular)

has a tightly associated ATP or ADP

they polymerize head to tail to form a right handed helix called a filamentous actin (microfilaments) or F actin

filaments are polar with a slower growing minus end and a faster growing plus end

important in establishing myosin movement relative to actin

Question 3

Assembly of actin polymers

Answer 3

small oligomers are unstable until they form an initial aggregate (3 monomers)

3 phases: nucleation, elongation, steady state

Question 4

Process of assembly and dissasembly

Answer 4

actin has a bound ATP and it is hydrolyzed to ADP and Pi following assembly

ATP not necessary but ATP makes the process much faster

can also depolymerize as needed

an equillibrium exists between actin monomers and filaments and is dependant on the concentration of free monomers

Question 5

Critical concentration

Answer 5

rate of actins polymerization into filaments is equal to the rate of dissociation

this is considered equilibrium

Question 6

equation of polymerization of Actin monomers

Answer 6

k off = C x k on

C=the concentration of free monomers

k on is dependant of the concentration and that is the rate of association

k off is independant of the C and is the rate of dissociation

Question 7

what makes up the 3 D structure of Actin filaments

Answer 7

THe actin filaments are organized into higher order structures that are regulated by actin binding proteins

Question 8

Where are actin filaments particulary abundant

Answer 8

beneath the plasma membrane
where they provide mechanical support, detemine the cell shape, and allow movement of the cell surface allowing for cells to migrate and engulf particles and divide

Question 9

What are three cross linking proteins of actin filaments

Answer 9

alpha-actin
filamin
spectrin

Question 10

Actin bundles

Answer 10

cross link into closely packed parallel arrays
polarity of actin filaments

consist of Parallel bundles
and contractile bundles

Question 11

Parallel bundle

Answer 11

closely spaced actin filaments in parallel

fimbrin monomer binds to actin filaments

has 2 actin binding domains

holds two parrallel filaments close together

increases the cell surface area allowing for additional receptors and channels which facillitated signaling transport and uptake of nutrients

supports projections of plasma membranes like microvilli

Question 12

Contractile bundles

Answer 12

loosely bundled actin filaments

alpa-actin
this is a bigger protein so it allows for the filaments to be seperated by a greater distance

allows motor protein to interact during contraction

contractile ring used in mitosis

Question 13

filamin

Answer 13

binds actin as a dimer and helps create a 3D meshwork

present in cells that need to withstand forces

Question 14

filopodia

Answer 14

thin projections of the plasma membrane supported by actin bundles

Question 15

Lamellipodia

Answer 15

broad sheet like extensions at the leading edge of cell, containing a network of actin filaments

Question 16

Pseudopodia

Answer 16

based on actin filaments cross linked into a 3-D network, responsible for phagocytosis

Question 17

Erythrocytes

Answer 17

have a strong cytoskeleton consisting of spectrin, ankrynin, and protein 4.1

important for the three dimensional shape of the erythrocyte

Question 18

Hereditary Spherocytosis

Answer 18

mutation in the erythrocyte cotrical cytoskeleton proteins: Spectrin, ankyrin, Protein 4.1

impaired deformabillity

makes the RBCs spherical leading to membrane breakdown and Damge to RBCs

not bi-concave

anemia, jaundice, splenomegaly

Question 19

Myosin

Answer 19

Motor protein

move along actin filaments via ATP hydrolysis

Skeletal Muscle Myosin II

Question 20

three major parts of Myosin

Answer 20

Head: contains the actin binding and ATP binding sites, also has ATPase activity

Neck: Flexible region
binds myosin light chain peptides

Tail: intertwine to bring myosin head regions in close proximity
bind membrane/organelles

Question 21

Myosin I

Answer 21

has only one head
one heavy chain
one light chain

function is for membrane association, and endocytosis

Question 22

Myosin II

Answer 22

2 heavy chain
2 light chain

used for contraction (muscle)

Question 23

Myosin 5

Answer 23

used for organelle transport

2 heavy chain
6 light chain

at the tail has a receptor that binds to a signal on a organelle

Question 24

Process of movement of the myosin along F actin

Answer 24

1) Binds ATP and the head dissociates from actin
2) hydrolysis of ATP to ADP and Pi and the myosin cocks its head
3) Myosin binds the actin filament
4) release of the Pi causes a straightening of the bound myosin causing the power stroke moving the myosin
5) ADP released and the whole process restarts

Question 25

WHat is the velocity of the powerstroke based on

Answer 25

velocity of the movement is proportional to the length of the neck longer neck the faster the movement

Question 26

THick filaments composed of:

Answer 26

6 myosin polypeptide chains

Question 27

THin filament composed of

Answer 27

actin, tropomyosin, and troponin

Question 28

important proteins in sarcomere

Answer 28

CapZ- binds the actin on the plus end Tropomodulin- bind to the minus end of the actin Nebulin- caps the minus end to prevent disassembly Titin- gives more contractillity

Question 29

Contraction of smooth muscle

Answer 29

similar to skeletal but the smooth doesnt have tropomyosin but has calmodulin calcium binds calmodulin which causes a conformational change which will bind to myosin light chain and then will phosphorylate to the active myosin light chain and that will cause the power stroke dephospho form does not

Question 30

Non muscle cell contraction

Answer 30

much less stable or organized cytokinesis bundles of F actin and myosin form contractile ring alpha actinin myosin moving along causes the pinching

Question 31

Dystrophin

Answer 31

links the cytoskeleton of muscle fibers to the surrounding connective tissue and the extracellular matrix this interaction acts as a shock absorber stabilizes the sarcolemma connects the PM to the actin filaments

Question 32

Duchene Muscular Dystrophy

Answer 32

X linked recessive disorder abnormal dystrophin progressive muscle wasting wheelchairs by age 12 and die of respiratiry failure by age 22 Milder form with later onset Becker muscular dystrophy

Question 33

how is dystrophin disrupted

Answer 33

DMD are out of frame mutations- little to no expression of dystrophin BMD: in frame mutation- smaller protein with partial function large internal deletions and lead to muscle degeneration gene therapy dystrophin replacement