exam 3 cytoskeleton Flashcards
what is the cytoskeleton
a system of protein filaments that provide structure and mechanical support for the cell
what are the three main types of fibers in the cytoskeleton
microbtubules (polymers of tubulin), microfilaments (of actin), intermediate filaments (of helical proteins)
what are cytoskeletal filaments made up of
smaller repeating subunits that self-assemble into large fibers
what are actin and tubulin fibers polarized with
positive and negative ends, where polymerization and depolymerization occur asymmetrically
what do motor proteins allow for
allow cargo to be transported along cytoskeletal fibers and can also be used to generate large cellular shape changes
what is the size scale of cytoskeletal fibers
microtubules are biggest, intermediate filaments are medium size, microfilaments are smallest in diameter
what are cytoskeletal structures constructed by
the polymerization of monomeric protein subunits
what does disassembly and reassembly allow for
changes in cell shape and/or internal movements of organelles/vesicles
what does the polymerization of cytoskeletal monomers require
nucleoside triphosphate in the form of either GTP (tubulin) or ATP (actin) (together = NTP)
what cytoskeletal monomers have a higher affinity for their binding partners
containing NTP over NDP
what occurs at the plus end of the cytoskeleton
addition if NTP bound
what occurs at the minus end of the cytoskeleton
release if NDP bound
what is actin composed of
a network of flexible filaments dispersed throughout a cell - highly concentrated beneath the plasma membrane (called the “cortex”)
what does actin form
the basis of cell shape and structure and the contractile rings of dividing cells
what does actin do
aids in the contraction of muscle cells and propels vesicles and other cellular compartments through the cytoplasm
what are the actin monomers
globular actin (G actin), which polymerizes into actin polymers (F actin) - two strands of actin wind around each other to form intact microfilaments
where are actin monomers bound to ATP added
to the plus end of the growing filament (ADP monomers are lost from the minus end)
what is treadmilling
dynamic instability actin exhibits where addition at positive end is equal to removal at minus end
what are the ways tubulin/actin is changed
increase nucleation, depolymerization
what do the rho family of GTPases do
act as molecular switches to control actin polymerization dynamics by regulating the activity of actin-binding accessory proteins
what does Rho-GTP do
regulates actin binding and stress fibers
what does Rac-GTP do
regulates actin polymerization and can generate structures of lamellipodia and membrane ruffles
what does Cdc42-GTP do
regulates actin polymerization and bundling, filopodia, and microspikes
what do microtubules form
- a network of rigid tubules that radiate through the cytoplasm of all eukaryote cells
- mitotic spindles of dividing cells
- the core of motile appendages: cilia and flagella
what are microtubules formed from
alpha and beta tubulin (both bind GTP, but only beta hydrolyzes GTP)
how does the plus end of microtubules grow
by the addition of tubulin dimers bound to GTP
where are tubulin dimers lost
from the minus end of microtubules, which grows slow and contains more GDP
when is alpha-beta tubulin bound to GDP/GTP
GDP at minus end, GTP at + end
how does the plus end maintain a GTP cap
rate of polymerization at plus end is more rapid than rate of GTP hydrolysis
when is the GTP cap lost
when the rate of GTP hydrolysis exceeds the rate of polymerization
what is catastrophe
when the plus end undergoes rapid depolymerization
what is NFAT
a transcription factor for activating immune system players regulated by localization - part of calcium signaling pathway; regulated by phosphorylation/dephosphorylation
how is actin asymmetric and directional
actin monomer assembles in directional way so one end points toward plus end and other points toward minus of microfilament - add on plus end, disaffiliate on minus end
how is tubulin different from actin
minus end of tubulin is fixed and we don’t get loss of GDP bound monomers from minus end
how is actin dynamic at both ends
add at plus end and subtract at minus end - size of filament is based on simultaneous rates of addition and subtraction
what can the growth or shrinkage of microtubules be regulated by
altering the balance between the addition and removal of tubulin dimers
how can you stabilize microtubules
- binding subunits to prevent assembling into microtubules
- enhancing GTP hydrolysis to enhance disassembly
- severing microtubules
- linking + ends to other structures
- stabilizing + ends of tubules
- binding along side tubules to stabilize them
what are the two phases of microtubule assembly
- nucleation: small portion of tubule formed at the beginning
- elongation: addition of tubulins and the GTP-cap
what are microtubule organizing centers (MTOC’s)
two barrel-shaped centrioles surrounded by centrosome matrix, which each contain nine fibrils, each composed of three microtubules
what can be used to generate directional motion
protein shape changing
how does movement become directional in order to be useful
couple shape change in one direction with ATP hydrolysis - makes an irreversible step when hydrolyzed
in which direction is movement favorable
ATP -> ADP + Pi
how is motion generated
- the protein starts in one position
- the binding to ATP causes a shape change where the protein takes a step forward
- ATP is hydrolyzed, which makes for another step forward.
- can release ADP and phosphate to bring protein back to original conformation, but it has taken a step forward at that point
what are myosins
a huge family of motor proteins that bind to actin microfilaments
what is the myosin II structure
heteromer with 6 polypeptide chains - one pair of heavy and two pairs of light chains
what does each heavy chain of myosin II contain
a “S1” head with ATP-ase activity, a neck region, a coiled-coil tail
what can myosin II molecules associate into
filaments, which form a basic structural unit of the contractile machinery
how can myosin II move actin filaments
by attaching to actin filaments, moving with the release of ADP and then detaching when ATP binds - myosin moves from - to + end
what are kinesins
microtubule-associated motor proteins
what do kinesins do
motor protein responsible for moving vesicles and organelles along nerve axons from the cell body to the synaptic terminals
what are kinesins composed of
two light and two heavy chains - heavy chains are entwined to create a stalk region made up of coiled alpha-helices
what do the two heads of the heavy chains of kinesins have
ATP-binding sites and bind to microtubules initiating ATPase activity and the movement of the kinesin molecules and cargo along the microtubule
what are the lagging and leading heads bound to in kinesins
lagging head is bound to ATP, leading is bound to ADP - hydrolysis allows for “walking” - moves - end to + end of microtubules
what are dynesins
a family of microtubule based motor proteins
how can kinesins interact with organelles
light chains in tail can interact with the cargo and the different types go to different locations in cell
how can cargo be loaded on and walk along microtubule
adaptor proteins interact with kinesins to allow for this
what is the structure of dynesins
dimer of heavy chains with two ATP-binding heads and a stalk (with intermediate and light chains)
what is dynactin
a complex of proteins that light chains can bind to a cargo through
how do dynesin molecules move along microtubule protofilaments
in a similar manner to kinesin, from the + end to the - end of the microtubule
how can cells regulate which direction something is moving in
by whether it asociates with dynamin or kinesin
what involves hydrolysis of ATP
depolymerization of actin filaments, movement of myosin along an actin fiber, movement of kinesin along a tubulin fiber
what are intermediate filaments composed of
a group of related long helical proteins
what do intermediate filaments do
provide mechanical strength to cells
how are intermediate filaments formed
via coiled-coil interactions of alpha helical proteins
what do lamins do***know!!
intermediate filament that provides structural support to nucleus
what do keratins do***know!!
intermediate filament that provides strength to epithelial cells
what are axons***know!!
intermediate filament that provides strength for long, extended neurons
what are actin filament-/microtubule affecting drugs
drugs that bind cytoskeletal proteins and affect their assembly/disassembly dynamics
what are the actin filament–affecting drugs
- latrunculin - binds G-actin monomers and prevents assembly of filaments
- phalloidin - binds F-actin filaments and prevents disassembly
- cytochalasins - bind + ends and suppress filament dynamics
what are the microtubule-affecting drugs
- nocodazole - binds tubulin dimers and prevents microtubule assembly
- taxol - binds to microtubules and stabilizes them to prevent disassembly/ inhibit cell division
- colchicine - copolymerizes into the microtubule lattice, suppressing microtubule dynamics
