Final: Ch 17 Actin Flashcards
microvilli
fingerlike projections that increase the area of the plasma membrane available for nutrient absorption
non-motile, made of actin
lining of digestive system
is the internal motile machinery of macrophages and other crawling cells always oriented int he direction they crawl?
yes
cell polarity
the ability of cells to generate functionally distinct regions
ex. cell division
cytoskeleton
a 3D filamentous protein network found in all eukaryotes
gives a cell shape, organization, and polarity
the cytoskeleton is composed of _ major filament systems
3
microfilaments, microtubules, intermediate filaments
the monomers undergo assembly and disassembly, allowing the cell to assemble or disassemble structures
microfilaments
polymers of the protein actin
can function on their own or serve as tracks for ATP powered myosin motor proteins
microfilaments are especially important in the….
organization of the cell membrane
gives shape to surface structures like microvilli
cell division
myosin motor proteins
provide a contractile function (like in muscle)
ferry cargo along microfilaments
microtubules
long tubes made of the protein tubulin
movement within the cell
function of microtubules
organizational framework for organelles
structural support for cilia and flagella
mitotic spindle
kinesins and dyneins
molecular motors that transport cargo along microtubules
powered by ATP hydrolysis
intermediate filaments
tissue specific filamentous structures
less dynamic, more structural
functions of intermediate filaments
structural support to nuclear membrane
structural integrity to cells in tissues
structural and barrier functions in skin, hair, and nails
to establish arrangements of the cytoskeleton, cells must sense ______, and interpret them
signals – cell-surface receptors detect and activate signal-transduction pathways
soluble factors bathing the cell
adjacent cells
ECM
how does cell motility work (microfilaments)
polymerization at the front of the cell
contractions at the back of the cell
macrophage (WBC) motility
move out of blood vessels and crawl through tissues to sites of infection
neural crest cell motility in embryos
from dorsal midline to development of the face
cell motility in neurons
growth cones at the ends of axons and dendrites move outward
chemotaxis definition
moving in response to external signals
microfilaments can assemble into a wide variety of _______ within a cell
structures
cell cortex
a less ordered network of microfilaments under the plasma membrane
support and organization
adherens belt
a contractile band on the outside of epithelial cells to form a seal between cells
made of microfilaments
associated with adherens junctions to provide strength
lamellipodium/leading edge
a network of microfilaments found in migrating cells at the front of the cell
may also have protruding bundles of microfilaments called filopodia
stress fiber
contractile microfilaments attached to the external substratum via focal adhesions/contacts
macrophages use __________ ___________ during phagocytosis to engulf and internalize pathogens
contractile microfilaments
short bursts of ____ ________ assembly can power the movement of endocytic vesicles away from the plasma membrane
actin filament assembly
contractile ring
polymer of actin and non-muscle myosin connected to cell membrane
constricts to form 2 daughter cells in the process of cytokinesis (late cell division)
actin
basic building block of microfilaments
a protein that can reversibly assemble into a polarized filament with functionally different ends
actin-binding protein
assembles actin into various structures
what does the name microfilament mean
actin in its polymerized form with associated proteins
actin is _____, ______, and highly _______
ancient, abundant, highly conserved
multiple actin genes in eukaryotes are related to a bacterial gene that evolved to have a role in bacterial cell-wall synthesis
_ types of actin isoform
3
alpha-actin
beta-actin
gamma-actin
what functions are associated with the 3 isoforms of actin
alpha-actin: contractile structures
beta-actin: cell cortex and leading edge of motile cells
gamma-actin: filaments in stress fibers
_-actin monomers assemble into long, helical _-actin polymers
G-actin, F-actin
G-actin
globular monomer
F-actin
filamentous polymer made of G-actin monomers
each actin molecule contains…
a Mg ion complexes with either ATP or ADP
the G-actin monomer is separated into to lobes by a deep ____
cleft
base of the cleft contains the ATPase fold (Mg and ATP are bound here)
floor of the cleft is a hinge
adding cations of a solution of G-actin does what
causes it to polymerize into F-actin filaments
reversible (when concentration of ions is lowered)
F-actin has ______ and _______ polarity
structural, functional
all subunits in an actin filament are oriented….
the same way
polarity of an actin filament
one end differs from the other
( + ) end gets actin monomers added (grows)
( - ) end has actin monomers removed (loses)
what can be added to the ends of actin filaments to stabilize?
CAP proteins
ex. actin in striated muscle (no treadmilling)
does addition of actin happen faster in the front or the back?
front ( + ) end
polymerization of G-actin to F-actin requires what
energy
ATP –> ADP
myosin binds specifically to…
actin filaments
myosin S1
the actin-binding head domain of myosin
decorating experiment (w/ electron micrograph)
an excess of myosin S1 (binds actin heads) is mixed with actin filaments
when all the actin subunits are bound by myosin, the filament appears “decorated” with arrowheads that all point toward one end of the filament
what have decorating experiments allowed researchers to determine
the polarity of actin filaments in vitro and in cells
the arrowhead points to the ( - ) end (pointed end)
the ( + ) end is known as the barbed end
actin polymerization takes _ steps
3
nucleation, elongation, steady-state
nucleation phase
a lag period where G-actin subunits combine into 2-3 subunits
when the oligomer reaches 3 subunits it acts as a nucleus for the elongation phase
elongation phase
short actin oligomer adds actin monomers to both ends
as F-actin filaments grow, the concentration of G-actin monomers decreases until equilibrium is reached
steady-state phase
G-actin monomers exchange with subunits at the F-actin filament ends
no net change in filament length
critical concentation Cc
the concentration at which filaments are formed (must be higher)
the rate of addition of ATP-G-actin is nearly __ times faster at which end of the filament?
10 times faster at the + end
what is the rate of addition of ATP-G-actin dependent on?
the concentration of free ATP-G-actin
is the rate of subunit loss from each end similar?
yes
not dependent on the free ATP-G-actin concreation
is there a lower critical concentration at the + end? what does this mean
yes, means that rate of addition of ATP-G-actin to + end is faster
treadmilling (steady-state)
polymer is in dynamic equilibrium where it is being polymerized and depolymerized at the same time
ATP-actin is being added faster at the + end than - end, and ADP-actin is being removed from the - end – hence treadmilling
is their treadmilling in muscle fiber?
no
what is the ability of actin filaments to treadmill driven by?
ATP hydrolysis
as ATP-actin is bound to + end, ATP-actin is hydrolyzed to ADP-Pi-actin at the - end (Pi gets released)
actin-filament treadmilling is accelerated by ______ and _______
profilin and cofilin
what does profilin do
a small protein that binds ADP-actin on the opposite side of the nucleotide binding cleft
when it binds ADP-actin, the cleft opens and enhances the loss of ADP, which is replaced by ATP
profilin-ATP-actin binds + end, and profilin dissociates
what does cofilin do
small protein that binds F-actin-ADP subunits near - end
bridges two actin monomers and twists the filament to break it into small pieces
released ADP-actin is recharged to profilin and added to the + end
what regulates profilin and cofilin
signal transduction pathways
thymosin beta-4
an actin monomer sequestering protein that binds ATP-G-actin
inhibits ATP-G-actin’s addition to the filament
provides a reservoir of actin for polymerization
capping proteins do what
block assembly and disassembly at both ends of the actin filament
CapZ binds + end
tropomodulin binds - end
where is tropomodulin found
in cells where actin filaments need to be highly stabilized
ex. RBC, muscle
gelsolin
a cap protein ( + ) end that can sever the filament
regulated by increasing levels of Ca
2 examples of a actin nucleating proteins
Arp2/3 and formins
nucleate actin assembly via signal transduction pathways
integrin
cell surface transmembrane protein that binds specifically to the ECM
how do integrins work
hold onto the cell but lets go at the back, like tracks on a tank
forward motion comes from the back – like squeezing toothpaste
are integrins recycled?
yes, to the front by endocytosis
microtubules and motor proteins move the integrin vesicles to the front
actin is depolymerized by
cutting the polymer
