lec 6- cytoskeleton: actin 3 Flashcards
(40 cards)
what is the only inhibitor of the Arp 2/3 complex?
Arpin
what is Arpin? where does it localize? what activates it?
-a protein encoded by a single gene
-localizes to lamellipodia
-is activated by Rac
how was Arpin discovered?
-the acidic “A” domain of WAVE is the part that binds to the Arp 2/3 complex, in WAVE there is a specific Tryptophan in the 3rd from the last position of the Acidic domain
-they use biomechanics to look for another protein that had the exact same A domain and found an uncharacterized protein
if Arpin has the same A domain, why does it not activate Arp 2/3 complex?
-because it lacks the V and C homology domains which are needed to activate the Arp 2/3 complex
when Arpin is present or blocked, what happens to the Arp 2/3 complex?
present: inhibits Arp 2/3 complex, directional changes
blocked: very active Arp 2/3 complex and straighter movements
does Arpin influence chemotaxis (whole cell migration)?
no, cells with Arpin knocked out still move the same speed and distance as cells containing arpin
what is the structure of actin within lamellipodia? what is it capped by? how does it move?
-it elongates 50-500 subunits/sec
-pushes against the plasma membrane
-filaments are capped by capping protein at 1 micron in length
-moves in waves backward, called retrograde flow
what disassembles the filaments once at the back of the lamellipodium?
cofilin
what are the steps of actin polymerization at the lamellipodia?
- pool of ATP-actin bound to profilin in the cell
- signals activate WASp/Scar proteins
- WASp/Scar activates Arp 2/3 complex to initiate new filaments as branches on old filaments
- elongation
- growing filaments push membrane forward, wiggling allows for more monomers to squeeze in
- capping protein terminates elongation
- ATP hydrolyses and Pi dissociates
- ADF/cofilin severs and depolymerizes ADP-actin filaments
- LIM-kinase inhibits ADF/cofilin
do cells need the main body to move, or can they move with just the lamellipodia?
they only need the lamellipodia
what are filopodia? what are they thought to do?
-actin rich structures alongside microspikes that are in the lamellipodia
-membrane projections
-thought to be used to sample the cellular environment and direct the cells movement by guiding the lamellum, guide cell motility
how is the actin arranged in filopodia?
-the actin is arranged in parallel arrays and is highly crosslinked
what causes crosslinks?
actin binding proteins
what does filopodia start out as?
-microspikes in the lamellipodia, that eventually emerge out of the membrane to become filopodia
what are microvilli?
-finger like projections of membrane used to increase surface ares by 20x for absorption
what is the structure of microvilli? where are the barbed ends?
-each is 0.08-2 microns wide
-actin dense core of 20-40 parallel and cross linked actin filaments
-barbed ends are up towards the cell periphery
actin intermingles with the cortical actin
what are the 5 actin bundling proteins?
-Fascin
-Fimbrin/Plastin-1
-Espin
-Villin
-alpha-actinin
how many isoforms of fascin?
-3 isoforms (fascin 1, 2, and 3)
where is fascin found?
at parallel actin-rich structures at the cell periphery (microspikes, filopodia)
how is fascin controlled?
through phosphorylation by protein kinase C (PKC)
what happens when fascin is phosphorylated by PKC?
you get a loss of actin bundling
are KO mice fine when fascin is phosphorylated?
yes, only minor defects
how is the loss of actin bundling caused by phosphorylation of fascin reversed?
by the Rac G-protein that acts on the Fascin-PKC complex
who discovered fimbrin?
Anthony Bretscher
