cytoskeleton: actin filaments

Question 1

structure of an actin filament

Answer 1

two stranded helical polymers of protein actin.
flexible structures

Question 2

how are AFs organised?

Answer 2

variety of linear bundles, dispersed throughout cell. highly concentrated in cortex beneath membrane

Question 3

actin polymerisation can produce pushing forces

Answer 3

• polymerisation at front of the cell pushes leading edge forward
• phagocytosis; formation of pseudopods
• intracellular movement of cell to cell spread of pathogens

Question 4

nucleation

Answer 4

• catalysed by complex of proteins inc ARP
• differences on sides and minus end prevents ARPs from forming filaments on their own/with actin

Question 5

ARP

Answer 5

actin related protein
- nucleates AF growth from -ve end allowing rapid elongation along +ve
- can attach to side of another AF while remaining bound to -ve end of filament that has been nucleated

Question 6

ARP more benefit when

Answer 6

nucleates filaments when bound to side a pre-existing AF = filament branch that grows at 70 degree angle relative to OG filament

Question 7

formins

Answer 7

• formation of actin bundle (opposed to gel like actin network)
• nucleates growth of straight, unbranched, filaments that can be cross linked by other proteins to form parallel bundles
• dimeric proteins and each subunit has binding site for actin monomer

Question 8

how is filament elongation modified?

Answer 8

by proteins that bind to free subunits

Question 9

soluble actin

Answer 9

not polymerised into filaments although conc of soluble actin is high

Question 10

how are actin monomers recruited by cells?

Answer 10

• depends on profilin

Question 11

what does profilin do?

Answer 11

binds to face of actin opposite ATP binding cleft
- actin-profilin can bind to + end if AF but unable to bind to the -end

Question 12

proteins that bind to the sides of actin

Answer 12

can stabilise or destabilise them

Question 13

tropomyosin

Answer 13

stabilised AF by bindingg simultaneously to 7 adjacaent actin subunits in one filament, prevents other proteins from binding

Question 14

cofilin

Answer 14

destabilises AF by forcing it to twist more tightly

Question 15

movement of listeria monocytogene

Answer 15

pathogenic bacterium that colonises the epithelial cell of the gut
- found in contamintated dairy
- infections lethal to babies or immuno-compromised people

Question 16

myosins

Answer 16

actin based motor proteins
- convert ATP hydrolysis into movement along AF
- >30 myosin classes in humans
- some myosin move cargo, others slide
- actin and atp binding sites in N-terminal head domain