lecture 5 Flashcards
what does the leading edge contain?
branched (dendritic) network of actin filaments
whats the formula for the net growth at one end of an actin polymer?
net growth= subunits added-subunits lost
=Kon x (concentration of actin)- koff
since Koff is the same regardless of concentration of actin, what is Koff for the + and - end?
+ end: 1.4/s
- end: 0.8/s
which actin regulators work together to drive forward motion of the cell?
-cofilin (cuts the filament)
-capping protein
-Arp2/3 (make branched network)
true or false, cross linkers have 2 opposing filament binding domains
true
what does a listeria infection do to a human cell?
hijacks the actin machinery of human cells
will make a structure out of the actin, to propel into the membrane to infect other cells
what are listeria comet tails nucleated by?
Arp2/3
what are comet tails?
propulsions of the bacteria
its a bacteria with a long actin tail
what does Arp2/3 do to form comet tails?
arp2/3 mimics an actin nucleus and nucleates daughter filaments at a 70 degree angle. It acts as a nucleus because forming an actin nucleus is energetically unfavorable, as it requires the assembly of 3 actin polymers together
what are the 3 distinct phases in which actin is polymerized?
-nucleation (before polymer can grow, 3 actin molecules need to come together to form the nucleus)
-elongation
-steady state
why do we need the 70 degree angle
its the optimal angle for pushing out the plasma membrane
what do microvili and stress fibers contain
bundles of actin filaments
what activates Arp2/3?
Wasp proteins
how is the listeria able to activate Arp2/3, in order to hijack the actin system of the cell?
they are coated with ActA, a protein that mimics WAsp
what are the 3 domains of a WASp protein?
W, C, A domains
which part of the WASp protein binds to actin?
WH2 domain
which domain of the WASp protein is acidic? what does it do?
-the yellow A domain
-this acidic domain induces change in ARP2/3 which activates it
which WASp is the best? which is the worst?
WASP AND WASP WCA
WASP CA
what challenges does the lamellipodium need to overcome?
1) running out of actin
2) preventing futile polymerization
what exactly is the lamellipodium?
The lamellipodium is a sheet-like, actin-rich protrusion at the leading edge of a migrating cell. It is essential for cell movement, especially in processes like wound healing, immune responses, and cancer metastasis. The lamellipodium is formed by the polymerization of actin filaments, creating a dense, branched network that pushes the cell membrane forward.
what is futile polymerization?
polymerization of ends not in contact with the plasma membrane, and depolymerizes before it can contribute to cell movement
how does profilin help lamellipodium overcome challenges?
how does it do this?
recycles actin and blocks minus end polymerization
-recycles actin by binding to ADP-G-actin, and recruiting ATP to bind
-blocks minus end polymerization, because it binds to the part that the actin molecule would bind to the negative end
how does ADF/cofilin work?
it severs actin filaments
ADF/cofilin has a strong preference for older actin filaments where ATP has already hydrolyzed to ADP.
It binds along the filament, inducing a conformational change that loosens subunit interactions.
how do ADF/cofilin and profilin work together?
Cofilin releases ADP-actin monomers, but these need to be reactivated for polymerization.
Profilin binds the released ADP-actin, promotes ADP-to-ATP exchange, and delivers ATP-actin to the growing (+) end.
this accelerates actin treadmiling
why do we need cofilin and profilin to help with treadmiling?
treadmiling normally only occurs in certain concentrations of actin, but in cells high concentration of actin, therefore we need them to help out
what does thymosyn-B4 do? HOW DOES IT WORK WITH COFILIN AND PROFILIN
keeps ATP-actin in reserve
what do capping proteins do?
prevent futile polymerization
what do cap z proteins do?
Regulating Actin Filament Length
blocks the + end
By blocking filament growth, CapZ controls the size and density of actin networks, especially in structures like the lamellipodium and muscle Z-disks.
This ensures actin filaments do not grow uncontrollably.
what does tropomodulin do?
blocks the -end
what is listeria motility driven by
coordinated action of 6 proteins
when do capping proteins bind to the actin filament?
capping protein cant bind immediately to newly nucleated actin filament. takes a second, during that one second has time to push on membrane. will then get capped so that another branch can grow and push on the membrane.
true or false, capping proteins are only used in lammelipodium
false, they can be used in cytoskeleton, etc. If we want constant size of actin filament, we need capping proteins
what happens if we have insufficient capping?
fish bones (branches growing from sides, and not contributing to effective polymerization that will push on the membrane
