Cytoskeleton (1-3) Flashcards
Families of protein filaments
-actin filaments
-microtubules
-intermediate filaments
for cells to function properly, they must organize themselves in space and interact mechanically with each other and with their environment.
cytoskeleton
shape of the cell’s surface; whole-cell locomotion; pinching of one cell into two.
Actin filaments
positions of membrane-enclosed organelles; direct intracellular transport; from the mitotic spindle.
Microtubules
mechanical strength
intermediate filaments
-are dynamic and adaptable
-can change or persist, according to need
-a structural rearrangement in a cell requires extra energy when conditions change.
Cytoskeletal systems
-underlie the plasma membrane of animal cells.
-strength and shape to its thin lipid bilayer.
Actin filaments
cell-surface projections
lamellipodia and filopodia
on the surface of hair cells in the inner ear contain stable bundles of actin filaments that tilt as rigid rods in response to sound.
stereocilia
on the surface of intestinal epithelial cells vastly increase the apical cell-surface area to enhance nutrient absorption.
microvilli
In plant, ____ filaments drive rapid streaming of the cytoplasm inside cells.
Actin
-found in cytoplasmic array that extends to cell periphery.
-form a bipolar mitotic spindle during cell division.
Microtubules
-function as motile whips or sensory devices on the surface of the cell.
-tightly aligned bundles that serve as tracks for the transport of materials down along neuronal axons.
cilia
In plant cells, organized arrays of _____ help to direct the pattern of cell wall synthesis.
microtubules
in many _______, microtubules form framework upon the entire cell is built.
protozoans
-line the inner face of the nuclear envelope, forming a protective cage for the cell’s DNA.
intermediate filaments
in the ________, intermediate filaments twisted into strong cables that can hold epithelial cells sheet together or help nerve cells to extend long and robust axons, and allow to form tough appendages such as hair and fingernails.
cytosol
rapid reorganization of cytoskeleton
cell division
After the chromosomes have replicated, the ________ _______ array that spreads throughout the cytoplasm is reconfigured into the bipolar _______ _______.
interphase microtubule, mitotic spindle
enable the fibroblast to crawl across the surface of the dish rearrange so that the cell stops moving and assume a more spherical shape.
specialized actin structures
after replication: form a belt around the middle of the cell
actin and motor protein
constricts like a tiny muscle to pinch the cell into two.
contractile ring
a protrusive structure filled with newly polymerized actin filaments
neutrophils
produce stable, large-scale structures for cellular organization.
mature neurons or epithelial cells
specialized epithelial cells in the intestines and lungs
microvilli and cilia
able to maintain a constant location, length, and diameter over the entire lifetime of the cell.
microvilli and cilia
for the actin bundles at the cores of microvilli on intestinal epithelial cells, is only a ____ days.
few
stereocilia on the hair cells
lifetime
Cytoskeleton is also responsible for large-scale ______ _____, enabling cells to tell the difference between top and bottom or front and back.
cellular polarity
use organized arrays of microtubules, actin filaments, and intermediate filaments to maintain the critical differences between the apical surface and the basolateral surface.
polarized epithelial cells
also must maintain strong adhesive contacts with one another to enable this single layer of cells to serve as an effective physical barrier.
polarized epithelial cells
How does cells builds filaments?
by assembling large numbers of the small subunits, like building a skyscraper out of bricks.
can diffuse rapidly in the cytosol, whereas the assembled filaments cannot.
small subunits
subunits for actin filaments
actin subunits
subunits for microtubules
tubulin
subunits of intermediate filaments
smaller subunits;
symmetrical
form as polymeric assemblies of subunits that self-associate, using a combination of end-to-end and side-to-side protein contacts.
the three major types of cytoskeletal filaments
The subunits of actin filaments and microtubules are asymmetrical and bind to one another head-to-tail so that they all point in one direction.
Polarity
-are symmetrical and thus do not form polarized filaments with two different ends.
-also do not catalyze the hydrolysis of ATP or GTP.
Intermediate filament subunits
Microtubules are built of ___ protofilaments
13
linear strings of subunits joined end-to-end that associate with one another laterally to form a _______ ________.
hollow cylinder
the greater energy required to break multiple noncovalent bonds simultaneously allows microtubules to resist ______ _______.
thermal breakage
-determines the spatial distribution and the dynamic behavior of the filaments.
- bind to the filaments or their subunits to determine the sites of assembly of new filaments.
- bring cytoskeletal structure under control of extracellular and intracellular signals.
- maintain a highly organized but flexible internal structure.
accessory proteins
among the most fascinating proteins that associate with the cytoskeleton are the ______ ________.
motor proteins
Where does the motor protein bind?
polarized cytoskeletal filament.
uses the energy derived from repeated cycle of ATP hydrolysis to move along the filament, and the “cargo” they carry
motor proteins
what many motor proteins carry
membrane-enclosed organelles
What are the organelles that the motor proteins carry?
mitochondria, Golgi stacks or secretory vesicles to their appropriate location in the cell.
Other motor proteins cause _____ ______ to exert tension or to slide against each other, generating the force that drives such phenomena as muscle contraction, ciliary beating, and cell division.
cytoskeletal filaments
Bacteria has ______ of all eukaryotic cytoskeletal filaments.
homologs
-tubulin homolog –>forms Z-ring (septum during cell division)
-generate a bending force that drives the membrane invagination and site for localization of enzymes.
FtsZ
-actin homolog
-scaffold to direct the synthesis of the peptidoglycan cell wall
MreB and Mbl
abnormalities in cell shape and defects in chromosomes segregation
mutations
bacterial actin homolog; encoded by a gene on certain bacterial plasmids that also carry genes responsible for antibiotic resistance.
ParM
ParM assembles into filaments that associate at each end with a copy of the plasmid, and growth of the ParM filament pushes the ______ ______ ______ apart
replicated plasmid copies
-influences the crescent shape of Caulobacter crescentus
-homolog of intermediate filaments
Crescentin
actin subunit
globular or G-actin
How many amino acid polypeptide carrying a tightly associated molecule of ATP or ADP does the actin subunit globular or G-actin have?
375
three isoforms of actin
alpha (a), beta (B) and gamma (y) actin
isoform of actin that is expressed in muscle cells
a-Actin
isoforms of actin that are found together in almost all non-muscle cells.
B- and y-Actin
Assembly of actin subunits
head-to-tail–> tight, right-handed helix
Actin subunits assemble head-to-tail to form a tight, right-handed helix, forming a structure about 8 nm wide called ______ or ________.
Filamentous or F-actin
filaments are polar and have structurally different ends: a slower growing minus end or _____ ___
pointed end
filaments are polar and have structurally different ends: a faster-growing plus end or ______ ___.
barbed end
nucleotide-binding cleft directed toward the ______ ___.
minus end
the minimum length at which random thermal fluctuations are likely to cause it to bend.
persistence length
The regulation of actin filament formation is an important mechanism by which cells control their _____ and _______.
shape and movement
subunits assemble into an initial aggregate, or nucleus, that is stabilized by multiple subunit-subunit contacts and can then elongate rapidly by addition of more subunits.
nucleation
is rate limiting and is additionally inhibited by actin-binding proteins
formation of small actin oligomers
In vitro polymerization of G-actin proceeds in three sequential phases, what are these?
-nucleation
-elongation
-steady-state
lag period; G-actin aggregates into short, unstable oligomers; when oligomers reaches a certain length, it can act as a stable seed or nucleus.
Nucleation
rapidly increases in length by the addition of actin monomers to both of its ends.
elongation
as F-actin filament grows, the concentration of G-actin monomers ______.
decreases
G-actin monomers exchange with subunits at the filament ends, but there is no net change in the total mass of filaments.
steady-state
when steady-state phase has been reached, the concentration of the pool of unassembled subunits is called ______ ______, __
critical concentration, Cc
the ratio of the “on” and “off” rate constant
dissociation constant
measures the concentration of G-actin where the addition of subunits is balanced by the dissociation of subunits
dissociation constant
actin filaments grow faster at ____ ___ than at _____ ___.
plus end, minus end
manifested by the different rates at which G-actin adds to the two ends
polarity of F-actin
nucleate the polymerization of G-actin
myosin-decorated actin filaments
newly polymerized (undecorated) actin is ______ times as long at the (+) end as at the (-) end of the filaments.
5-10
difference in elongation rates at the opposite ends of an actin filament is caused by
a difference in Cc values at the two ends
the plus end of an actin filament is ______, it can elongate only from its minus end
capped
elongation takes place only at the plus end when the minus end of a filament is ______.
blocked
Cc is about ____ _____ lower for polymerization at the (+) end than for addition at the (-) end
six times
G-actin concentration below Cc+, there is __ ________ ______.
no filament growth
G-actin concentrations between Cc+ and Cc-
growth is only at the (+) end
G-actin concentration above
no growth at both ends
G-actin concentrations intermediate between the Cc values for the plus end the minus ends, subunits continue to be added at the end and lost from the minus end
steady-state phase
newly added subunits travelling through the filament, as if on a ______, until they reach the plus end, where they dissociate
treadmill
effect on filaments: Latrunculin
depolymerizes
effect on filaments: Cytochalasin B
Depolymerizes
effect on filaments: Phalloidin
stabilizes
Chemical inhibitors of actin
-Latrunculin
-cytochalasin B
-Phalloidin
chemical inhibitors of Microtubules
-Taxol (paclitaxel)
-Nocodazole
-Colchicine
effect on filaments: Taxol
stabilizes
effects on filaments: Nocodazole
Depolymerizes
effects on filaments: Colchicine
depolymerizes
Binds actin subunits
Latrunculin
Caps filament plus ends
Cytochalasin B
Binds along filaments
Phalloidin
Binds along filaments: Microtubules
Taxol
Binds tubulin subunits
Nocodazole
Caps filament ends
Colchicine
polymerization of actin is controlled by
its concentration, pH, and conc. of salts and ATP
actin behavior is regulated by _________ ________ that bind actin monomers or filaments
accessory proteins
a measure of how long an individual actin monomer spends in a filament as it treadmills.
filament half-life
50% of the actin is in _____ and 50% is
soluble ____
filament
monomer
cell contains proteins that bind to the actin
monomers and make polymerization much ___ ____
less favorable
inhibition of actin polymerization;
they cannot associated with either the plus
or minus ends; neither hydrolyze nor
exchange their bound nucleotide
thymosin
binds to the face of the actin
monomer opposite the ATP-binding cleft,
blocking the side of the monomer that would
normally associate with the filament minus
end, while leaving exposed the site on the
monomer that binds to the plus end
Profilin
binds to the face of the actin
monomer opposite the ATP-binding cleft,
blocking the side of the monomer that would
normally associate with the filament minus end, while leaving exposed the site on the monomer that binds to the plus end
profilin
prerequisite for cellular actin polymerization
filament nucleation
bring several actin subunits together to form a seed
actin-nucleating proteins
occurs primarily at the plasma membrane, and therefore the highest density of actin filaments in most cells is at the cell periphery within the cell cortex
Arp2/3 complex and formins
actin-related proteins
Arp2/3 complex
the complex can attach to the side of another actin filament while remaining bound to the minus end of the filament that it has nucleated
treelike web actin-nucleating factors
dimeric proteins that nucleate
the growth of straight, unbranched
filaments that can be cross-linked by
other proteins to form parallel bundles
formins
appears nucleate actin
filament polymerization by capturing
two monomers
formin dimers
strongly enhanced by the
association of actin monomers with
profilin
Formin-dependent actin filament
alter filament behavior
-bind along the side
-bind to the ends
actin filament-binding proteins
side-binding proteins; elongated protein that binds simultaneously to six or seven adjacent actin subunits
tropomysin
binds at the plus end; stabilizes an actin filament (inactive)
capping protein (capZ)
capping long-lived actin
filaments in muscle; minus end-binding
* binds tightly to the minus ends that have
been coated and stabilized by
tropomyosin
* reduce their elongation and
depolymerization
tropomodulin
coats the filament completely and present in high amounts
side-binding
affect filament dynamics
end-binding
- proteins that break an actin filament into
many smaller filaments; generating new
filament ends - newly formed ends nucleate filament
nucleation - promotes depolymerization of old filaments
severing proteins
One class of actin severing proteins
gelsolin superfamily
-activated by high levels of cytosolic Ca2+
-interacts with the side of the actin filament and contains subdomains that bind to two different sites
gelsolin superfamily
two different sites of gelsolin
-one that is exposed on the surface of the filament
-one that is hidden between adjacent subunits
a second actin filament-destabilizing proteins (severing proteins)
cofilin
-also called actin-depolymeriziing factor
-binds along the length of the actin filament, forcing the filament to twist a little more tightly.
cofilin
weakens the contacts between actin subunits in the filament, making the filament less stable and more easily severed by thermal motions, generating filament ends that undergo rapid disassembly.
Mechanical stress induced by cofilin
Cofilin binds preferentially to ____ _________ actin filaments rather than to ATP-containing filaments.
ADP-containing
actin filaments containing ATP are resistant to _________
depolymerization
Cofilin tends to dismantle the ______ filaments in the cell.
older
actin filament types of arrays
-dendritic networks
-bundles networks
-weblike (gel-like) networks
Arp 2/3 complex
dendritic networks
made of the long, straight filaments produced by formins
bundles networks
not -well defined
weblike (gel-like) networks
different actin networks depends on
specialized accessory proteins
cross-link actin filaments into parallel arrays
bundling proteins
hold two actin filaments together at a large angle to each other, forming a looser meshwork
gel-forming proteins
enable stress fiber and other contractile arrays to contract
myosin II
close packing of actin filaments; not contractile
fimbrin
cross-links oppositely polarized actin filaments into loose bundle; allowing the binding of myosin and formation of contractile actin bundles
a-actinin
formation of a loose and highly viscous gel; by clamping together two actin filaments roughly at right angles
filamin
defect in nerve-cell migration during early embryonic development
filamin A gene mutations
periventricular region of the brain fail to migrate to the cortex and instead form nodules
periventricular heterotopia
web-forming; long, flexible protein made out of four elongated polypeptide chains
spectrin
concentrated beneath plasma membrane; forms a two-dimensional weblike network
spectrin in RBCs
strong, yet flexible ____ ___ that provides mechanical support allow RBC to spring back its shape
cell cortex
it can form contractile structures through the action of myosin motor proteins
Myosin and Actin
-first motor protein identified
-generates force for muscle contraction
skeletal muscle myosin
an elongated protein formed from two heavy chains and two copies of each light chains
myosin II
globular head domain at its N-terminus. contains force-generating machine
heavy chain
amino acid sequence forming an extended coiled-coil
mediates heavy-chain dimerization
bind close to N-terminal head
light chains
oriented in opposite directions
myosin heads
-binds and hydrolyzes ATP
-use the energy of the ATP hydrolysis to walk toward the plus end of an actin filament
myosin head
result in powerful contraction
ATP-driven sliding of actin filaments
use structural changes in their ATP-binding sites to produce cyclic interactions with a cytoskeletal filament
motor proteins
propels them forward in a single direction to a new binding site along the filament
ATP binding, hydrolysis, and release
each step of the movement along actin is generated by the swinging of an 8.5 nm-long a helix
lever arm
a pistonlike helix that connects movement at the ATP-binding cleft in the head to small rotations
converter domain
sliding of myosin II along actin filament
muscle contraction
bulk of the cytoplasm inside is made up of ______
myofibrils
a cylindrical structure 1-2 um in diameter that is often as long as the muscle cell itself. It consists of a long, repeated chain of tiny contractile units
myofibril
-a long, repeated chain of tiny contractile units
-give the vertebrate myofibril its striated appearance
-parallel and partly overlapping thin and thick filaments
sarcomeres
actin and associated proteins; attached at their plus ends to a Z disc; minus ends overlapped with the thick filaments
thin filaments
hexagonal lattice, with actin filaments evenly spaced between the myosin filaments
thick filaments
caused by the myosin filaments sliding past the actin thin filaments, with no change in the length of either type of filament
sarcomere shortening
rise in cytosolic Ca2+ concentration initiates?
muscle contraction
two major features of the muscle cell
- myosin motor heads coupled binding and hydrolysis to ATP
- a specialized membrane system relays the incoming signal rapidly throughout the entire cell
signal from the nerves triggers an action potential
sarcoplasmic reticulum
triggers the opening of Ca2+ release channels in the sarcoplasmic reticulum
Ca2+ influx
elongated protein that binds along the groove of the actin filament helix
tropomysin
complex of polypeptide (T, I, & C)
troponin
pulls the tropomysin out of its normal binding groove that interferes with the binding of myosin head
troponin I-T complex
binds up to four molecules of Ca2+, causes troponin I to release its hold on actin
rise in Ca2+, troponin C
contraction is triggered by an influx of calcium ions, but different regulatory mechanism
in smooth muscle cells
elevated intracellular Ca2+ levels regulate contraction by a mechanism that depends on ________
calmodulin
Ca2+ bound calmodulin activates ____________, thereby inducing the phosphorylation of smooth muscle myosin on one of its two light chains
myosin light-chain kinase (MLCK)
phosphorylated light chains
contraction
dephosphorylated
inactive
most heavily worked muscle
heart
common cause of sudden death in young athletes
familial hypertrophic cardiomyopathy
genetically dominant inherited condition
-heart enlargement
-abnormal small coronary vessels
-disturbances in heart rhythm
encoding cardiac b myosin heavy chain or mutations in other genes encoding contractile proteins
point mutation in the genes
minor missense mutations in the cardiac actin gene
dilated cardiomyopathy
contain small amounts of contractile actin-myosin II bundles
non-muscle cells
-found in Acanthamoeba castellani
-intracellular organization - microvilli and endocytosis
myosin I
one-headed or two-headed
myosin family
how many distinct myosin families?
37
human genome includes ___ myosin genes
40
two-headed myosin with a large step size; organelle transport along actin filaments; move processively along actin filaments without letting go.
myosin V
actin cables in the mother cell point toward the bud, where actin is found in patches that concentrate where cell wall growth is taking place.
Saccharomyces cerevisiae
