AC Lecture 37: Intracellular architecture Flashcards
List the cytoskeleton framework filament types.
microtubules
intermediate filaments
actin/microfilaments (smallest)
Describe IF assembly.
monomer (elongated, alpha helical with a globular N-terminus and C-terminal tail) combine to form dimers (coiled coil) which associate to form tetramer which associates with 7 other tetramers to form ropelike filaments (associate head to tail so there is no polarity)
most are not reversible
What is the function of IFs?
protect the cell from stretching forces
surround nucleus to form a network towards the cell periphery
interact with desmosomes or hemidesmosomes (junction proteins)
What are the classes of IFs?
keratins
vimentins
nucleofilaments
nuclear lamins
What allows for the breakdown of the nuclear membrane during mitosis?
phosphorylation of nuclear lamins allowing for reversible assembly of intermediate filaments
What is the function of microtubules?
intracellular organization and intracellular transport of organelles and vesicles throughout the cell
form distinct structures such as the mitotic spindle, cilia, and flagella
What is the difference between cilia and flagella?
cilia- present on the exterior side of the plasma membrane to help move materials over the cell surface
flagella- larger and used for locomotion of whole cells (e.g. sperm)
Describe the structure of microtubules.
grow from a tubulin dimer (one alpha and one beta, each bound to GTP) forming linear protofilament that grow out of centrosomes from minus end (which binds to structures composed of alpha tubulin) to plus end (where there is a free beta to bind incoming alphas)
have polarity
Describe the dynamic growth of microtubules.
- alpha subunit binds to centrosome to start building
- under conditions where there are plentiful* free dimers, beta tubulin hydrolyzes its bound GTP as a free alpha subunit binds to it (minus to plus end)
- hydrolysis from GTP to GDP stabilizes structure
* note that under tubulin limited conditions, the GDP form is at the cap and hydrolyzes without the addition of another subunit to its end, destabilizing the structure
What drugs impact polymerization of microtubules?
prevent polymerization: colchicine/vinblastine
prevent depolymerization: taxol (chemotherapeutic)
What are microtubule binding proteins?
proteins that regulate the spacing between long tubules
What are the different kinds of microtubule-organizing centers (MTOCs)?
in interphase cells: centrosome
in cilia and flagella: basal body
What is dynamic instability?
the growing and shrinking of microtubules in a GTP hydrolysis dependent manner
when GTP is hydrolyzed before a new dimer can bind the end the structure is destabilized (dependent on the concentration of present dimers)
What motor proteins use microtubules as tracks?
kinesin (anterograde; plus end directed motors)- normally associated with synaptic vesicles and axonal growth materals
dyein (retrograde; minus end directed motors)- normally associated with recycling of membranes to lysosome in cell body
How are microtubles essential to cilia and flagella?
facilitate the beating motion via specific arrangements
also dependent on the action of motor proteins (dyneins induce slide of one microtubule against another)
What is primary cilia?
non-motile (due to lack of central pair of microtubules) sensory organs found in terminally differentiated cells
What diseases result from mutations in the function of cilia?
polycystic kidney disease
How are microtubules involved in cell division?
microtubules existing in interphase cells reorganize into the mitotic spindle which attaches to the centrioles
in prophase, centrioles divide and separate to opposing sides of the nucleus by kinesin motors (asters)
in prometaphase these become attached to centromeres via kinetochores to help organize and align the chromosomes in a congression during metaphase
during anaphase the chromosomes move apart under tension from teh kinetochore microtubules
What are the kinds of microtubules in the spindle.
kinetochore- microtubules that bind directly to the chromosomes at the centromeres
overlap/polar- bind each other in an overlapped fashion
astral- bind to the cell surface
How are microtubules in the spindle shortened through the cell cycle?
disassembly at both the kinetochore and spindle ends (motor proteins push and pull the spindle to move the poles further apart; dynamic instability constantly shrinks the kinetochore)
What is actin?
monomer protein that binds ATP and polymerizes to form actin filaments in a helical arrangement have polarity (plus end with G-actin binding; nucleotide facing minus end)
What roles do actin filaments play in human cells?
function dependent on acting binding proteins it interacts with:
form structural component of microvilli (seen in epithelial cells)
found around cell membrane (cell cortex)
form structural belt around the cell (adherens belt)
form structures used to promote cell movement or crawling (e.g. lamellipodia or filopodia) which can also promote endocytosis
form part of the contractile ring used in cell division
What are the steps of actin filament formation?
- nucleation
- elongation
- steady state
What is treadmilling?
where the rate of binding at the plus end of a growing actin filament equals the rate of dissociation at the minus end
