Slide set 11 Flashcards
cytoskeleton
- protein polymers composed of actin subunits, tubulin subunits, and intermediate filaments
microtubules are made up of
tubulin subunits
microtubule growth
microtubules continually grow from the centrosome added to a cell extract
dynamic instability
some microtubules suddenly stop growing and then shrink back rapidly (rapid disassembly)
constant polymerization and depolymerization cycle
actin filaments aka
microfilaments
actin filaments
- helical polymers of actin
- flexible
- organize into linear bundles, 2D networks, and 3D gels
- actin filaments are dispersed in the cell but most highly concentrated in the cortex (right below the plasma membrane)
where do you find actin filaments?
microvilli, striated msucle
microtubules
- long, hollow cylinders made of tubulin
- more rigid than actin
- long, straight
- usually have one end attached to a microtubule-organizing center (MTOC) called a centrosome
where do we find microtubules and what are they made of
tubulin!
cilia!
intermediate filaments
- ropelike fibers
- made of intermediate filament proteins
intermediate filament example
forms meshwork called nuclear lamina beneath the inner nuclear membrane
extend across cytoplasm
gives mechanical strength!
in epithelial tissue, they span cytoplasm from one-cell junction to another (strengthening epithelium)
cytoskeletal polymers are….
DYNAMIC!
- cells can rapidly reorganize cytoskeletal organization
- it uses subunits of polymers to build new structures
- cytoskeletal polymers assemble from subunits AND undergo self-assembly
cytoskeletal polymers determine…..
cell polarity and internal organelle organization!
image is small intestine
actin increases SA for food absorption
actin monomers bind together to form….
a polymer/filament!
- actin monomers have ATP binding pocket (ATP can be hydrolyzed to ADP)
- actin monomers have plus and minus end
- subunits bind together head-to-tail
- subunits are added to the end of growing polymer (NOT inserted in the middle)
polarity of actin filament image
barbed = plus end
pointed = minus end
2 Ps don’t go together!
blocking actin filament assembly/disassembly has what effect on cells
toxic!
actin chemical: Latrunculin
effect: depolymerizes
mechanism: binds actin subunits
source: sponges
L in latrunculin bc larry the lobster is in spongebob
actin chemical: Cytochalasin B
effect: depolymerizes
mechanism: caps filament plus ends
source: fungi
actin chemical: Phalloidin
effect: stabilizes
mechanism: binds along filaments
source: Amanita mushroom
actin assembly graph
A. polymerization of pure actin subunits into filaments occurs after a lag phase
B. polymerization occurs more quickly in presence of preformed fragments of actin filaments (act as nuclei for filament growth)
% of free subunits after polymerization reflects critical concentration (Cc) at which there is no net change in polymer
how do we study actin polymerization?
observe change in light emission from a fluorescent probe (pyrene)
fluorescent probe covalently attaches to actin
pyrene-actin fluoresces more brightly when incorporated into actin filaments
nucleation
- a helical polymer is stabilized by multiple contacts between adjacent subunits
- in actin, 2 molecs binds weakly to each other, but a 3rd actin (forms trimer) makes the whole group more stable
- once further monomer addition occurs, this now is a nucleus for polymerization. (tubulin nucleus is larger)
- assembly of nucleus is slow (explains the lag phase seen during polymerization)
- lag phase can be reduced or abolished entirely by adding premade nuclei (EX: fragments of already polymerized microtubules or actin filaments)
how do reduce or eliminate lag phase of actin polymerization?
lag phase can be reduced or abolished entirely by adding premade nuclei (EX: fragments of already polymerized microtubules or actin filaments)