Cytoskeleton (Microfilaments)

Question 1

Actin filament (microfilaments)

Answer 1

monomeric actin protein subunits, linked together and twisted into a double polymer

Question 2

Formation of microfilaments

Answer 2

1. nucleation = nucleus made
2. elongation = filament grows from both ends
3. steady state = both ends are continuously growing and shrinking

Question 3

Elongation formular

Answer 3

ATP-G-actin + filament –> APD-F-actin + Pi

~ exposed filament will disassemble (F-actin part)

Question 4

Critical concentration (Cc)

Answer 4

concentration of the free ATP-G-actin at which the assembly/disassembly are equal at one end
~ above Cc = end will grow
~ below Cc = end will shrink
~ different conc at +ve and -ve ends

Question 5

Treadmilling (def)

Answer 5

when ATP-actin subunits add to the +ve end and ADP-actin subunits leave the -ve end

Question 6

Treadmilling (about)

Answer 6

Occurs when
~ rate of addition = rate of loss at other end
~ actin sub. conc. higher than Cc at +ve end
+ actin sub. conc. lower than Cc at -ve end

Question 7

Actin binding proteins

Answer 7

regulate assembly and disassemble of microfilaments

Question 8

Profilin

Answer 8

~ actin binding protein

~ increases exchange of ADP –> ATP on G-actin

Question 9

Cofilin

Answer 9

~ actin binding protein

~ increase loss of ADP-actin from -ve end

Question 10

Capping proteins

Answer 10

~ actin binding protein
~ binds to filament ends, stops dis/assembly
CapZ = stabilises +ve end
Tropomodulin = stabilises -ve end

Question 11

Formins

Answer 11

nucleate the assembly of unbranched filaments
~ maintained in inactive state
~ Rho (in active, GTP-bound form) binds to formin = conformational change = active formin = dimer

Question 12

ARP 2/3

Answer 12

creates actin branching by sticking to the filament

~ sticks to filaments, gives them a bigger SA + increased mechanical strength

Question 13

Phalloidin

Answer 13

toxin from death cap mushroom
~ binds to f-catin and stops disassembly
~ if bound to fluorescent dye = stain

Question 14

Actin based cell structures + crosslinking proteins

Answer 14

~ organised by cross-linking proteins e.g.:
fimbrin = microvilli
filamin = filopodia
dystrophin = muscle cell cortex
Spectrin = cell cortex (maintains shape)

Question 15

Duchenne Muscular dystrophy

Answer 15

~ dystrophin gene (X chromo) is defective
~ adaptor protein that bind actin + dystroglycan
~ causes impaired muscle function and decreased lifespan

Question 16

Myosin

Answer 16

~ actin’s motor proteins
~ use ATP to ‘walk’ along actin filaments
~ 3 classes (I, II, V)

Question 17

Myosin I

Answer 17

~ single head
~ used in membrane association + endocytosis
~ travels -ve –> +ve

Question 18

Myosin II

Answer 18

~ short neck and two heads

~ bipolar complexes the work together = contraction

Question 19

Myosin V

Answer 19

~ long neck with two heads
~ globular cargo binding domain on tail (active only when cargo is bound)
~ used in organelle transport –> hand over hand model

Question 20

Skeletal muscle contraction is triggered by…

Answer 20

~ Ca2+ binding to troponin C
~ causes a conformational change in tropomyosin = exposes myosin-binding sites
~ Ca2+ release mediated by t-tubules in sarcoplasmic reticulum

Question 21

Smooth muscle contraction is triggered by…

Answer 21

myosin light chain kinase activation by Ca2+

Question 22

Contractile ring

Answer 22

made of actin and myosin which creates the cleavage furrow in cytokinesis