cytoskeleton - actin, microtubules

cytoskeleton functions

• move stuff around the cell

• keeps cell intact

  • keeps the cell shape

  • positions and moves organelles

• cell movement

cytoskeleton makeup

• actin monomers come together with hydrogen boding to form microfilaments

• tubulin dimers (alpha and beta) come together to form a hollow tube—microtubules

• intermediate filaments

• dynamic—change size quickly

• actin filaments outline the cell

• microtubules spread throughout (radiate from central hub)

• intermediate filaments are in lamin

cytoskeleton flexibilities

• actin filaments: not flexible, “steel”

• microtubules: somewhat flexible, “uncooked spaghetti”

• intermediate filaments: very flexible, “rubber bands”

cortical actin (actin)

• at/in the plasma membrane

• filopodia

• lamellipodia

• microvilli

stress fibers (actin)

• in the cytoplasm

• in bundles

• has myosin inside

G-actin

• free, disassembled actin

• actin monomers

• held together by hydrogen bonding??

F-actin

• structured, assembled actin

• actin filaments

• held together by hydrogen bonding???

nucleation (lag phase)

actin monomers starting to come together

elongation (growth phase)

growing actin filament

steady state (equilibrium phase)

• actin filament with subunits coming on/off

• add rate=loss rate: “caterpiller movement”

actin filaments are __ (polar/nonpolar) and grow more on _ (plus/minus) end than the (blank) (plus/minus) end

• polar

• plus

• minus

free actin __ (g-actin/f-actin) binds to ATP and turns into what form

• g-actin

• t-form

after polymerization, ATP is hydrolyzed into ADP so the actin is in what form

D-form

ATP hydrolysis causes conformational changes within subunits

T-form (actin with ATP) to D-form (actin with ADP)

T-form subunits

• bound to ATP

• bind more tightly with each other

  • favor polymerization

D-form subunits

• bound to ADP

• bind less tightly with each other

  • favor depolymerization

  • they’re in the middle so it makes sense

cells have no control over what

actin polymerization/depolymerization

thymosin

• binds to minus end of actin monomer

• now it (actin-thymosin) can’t bind to EITHER end of the actin microfilament

• promotes depolymerization

profilin

• binds to plus end of free actin monomer

• now it (actin-profilin) can only bind to the plus end of the actin microfilament

• promotes polymerization

profilin and thymosin what with each other

compete with each other

ARPs

• actin-related proteins

• act as stable bases for nucleation

• cap the minus end of microfilament so there’s only growth on the plus end

• localized to the cell membrane

ARP complexes

• laterally bind to actin filaments

• creates nucleating branch points and caps

formin

• binds to plus end of actin monomers

• enhances nucleation

• polymerization? formin rides on top of the growing end of the microfilament

formin + profilin

• SOME formins can bind with profilin

• close to the polymerization end of actin for easy access

• hands grabbing and slapping in on itself

actin cross-linking proteins

• link seperate actin filaments

• have 2 actin binding domains

• length between domains determines packing closeness which determines the structure of the microfilaments

• spectrin

• fimbrin

• alpha-actinin

• filamin

spectrin

• actin-cross linking protein

• tetramer

• super duper long

fimbrin

• actin-cross linking protein

• monomer

• super duper short

alpha actinin

• actin-cross linking protein

• dimer

• medium length

filamin

• actin-cross linking protein

• dimer

• “pants with 2 socks”

actin filaments + alpha-actinin = what

• stress fibers

• loose packing allows myosin to enter in the gaps between actin filaments

actin filaments + fimbrin = what

• filopodia

• tight packing doesn’t allow myosin or anything rlly to enter

actin filaments + filamin = what

• lamellipodia

• cross-linking actin filaments

actin filaments can be anchored to the what following the activation of linkers (like what)

• plasma membrane

• like ERM proteins

microtubule subunits are what

• dimers of alpha-tubulin and beta-tubulin that line up to create protofilament lines

• these lines circle around in order to make a microtubule cylinder with a hollow core (lumen)

microtubules and dimers are held together by what

hydrogen bonding

nucleation (microtubules)

• formation of a stable complex made up of a few dimers

• lets polymerization happen

microtubules are __ (polar/nonpolar) and add on what end

• polar

• plus

alpha-tubulin and beta-tubulin can bind what but only beta-tubulin can what it

• can bind GTP

• beta-tubulin can hydrolyze it into GDP

  • beta-tubulin is the only one with GDP

T-form tubulin

• free form tubulin dimers binded to GTP

• stabilizes binding

D-form tubulin

beta-tubulin bound to GDP after polymerization b/c GTP is hydrolyzed

catastrophe

• rate of hydrolysis > rate of polymerization

• GTP cap gets messed with (hydrolyzed)

• microtubule shrinks rapidly

rescue

• recovery of GTP cap and return of polymerization

• can happen after catastrophe

y (gamma) -tubulin (y-TuRCs)

• y-tubulin ring complex

• chemically mimic (+) end of a microtubule

• lets tubulin dimers bind

MTOCs

• microtubule organization centers

• ex: centrosomes

centrosomes

• organelles, MTOCs in animals

• composition of centrosomes

  • pair of centrioles

  • pericentriolar material

stathmin

• small protein

• binds to 2 tubulin dimers to where they can’t polymerize

• promotes depolymerization

MAPs

• microtubule-associated proteins

• bind to, brace, and stabilize microtubules

catastrophin

• kinesin 13

• destabilizes microtubules

• binds near the plus end

• uses ATP as energy and pries apart protofilaments

• opposite of what a MAP does btw

MAP2

• type of MAP

• has dual microtubule binding domains

  • they regulate microtubule compaction btw

• has a longer “arm” to space out the microtubules it connects to 

• creates less dense bundles of microtubules

• in cell body and dendrites

TAU

• type of MAP

• has dual microtubule binding domains

  • to regulate microtubule compaction btw

• has a shorter “arm” to pack microtubules closer together

• creates tighter bundles of microtubules btw

• in axons

why is TAU important in axons

• important for stabilizing the microtubules in there

• malfunctioning TAU?

  • unstable microtubules—might depolymerize

  • neurons might not work—alzheimers

microtubule severases and what do they do

• katanin

• spatsmin

• fidgetin

• cut microtubules

actin severase(s) and what do they do

• gelsolin

• cuts actin filaments

why sever microtubules

• accelerate the creation of new nucleation sites

• increase cytoplasmic fluidity

• regulate filament length

taxol

• a drug!!!!

• binds to beta-tubulin

• hyperstabilizes microtubules

• inhibits microtubule depolymerization

  • microtubules aren’t dynamic

phalloidin

• a drug !!!

• stabilizes actin filaments

latrunculin

• a drug !!!

• destabilizes actin filaments

colchicine

• a drug !!

• destabilizes microtubules