Cytoskeleton (4-6) Flashcards
highly dynamic and play
comparably diverse and important roles in the cell
Microtubules
Microtubules are polymers of the protein ?
tubulin
heterodimer formed from two closely related globular proteins called α-tubulin
and β-tubulin
tubulin
two tubulin proteins are
found only in this heterodimer
α-tubulin
and β-tubulin
has a binding site for
one molecule of GTP
α-tubulin and β-tubulin
Tubulin is found in all ____ cells
eukaryotic cells
give rise to a paralytic eye-movement disorder due to loss of ocular nerve function
mutations in human β-tubulin gene
mutations in a particular human β-tubulin gene
give rise to a
paralytic eye-movement
hollow cylindrical structure built from 13
parallel protofilaments
microtubule
microtubule is built from
13 parallel protofilaments
Each 13 parallel protofilaments of microtubule, composed of a?
αβ-tubulin heterodimers stacked head to tail and then folded into a
tube
along the longitudinal axis of microtubule
protein-protein contact
forms an interface with the “bottom” of the α-tubulin
molecule in the adjacent heterodimer
“top” of β-tubulin molecule
the main lateral contacts
α–α and β–β
the addition and loss of subunit occurs almost
exclusively at the
end
The subunits in each protofilament in a microtubule all point in the same?
direction
the subunits in each protofilament in a
microtubule all point in the same direction
structural polarity
exposed at the minus end
α-tubulins
exposed at the plus end
β-tubulins
influenced by the binding
and hydrolysis of GTP
microtubules dynamics
microtubules dynamics is influenced by the
binding and hydrolysis of GTP
occurs only within β-tubulin
GTP hydrolysis
bound GTP
“T form”
bound GDP
“D form”
two different types of microtubule structures
T form
D form
____- ____ tends to polymerize and ____-____ to
depolymerize
GTP tubulin
GDP-tubulin
whether the tubulin subunits at the very end of a microtubule are in the T or the D form depends on
the relative rates of
GTP hydrolysis and tubulin
addition
If the rate of subunit addition is high, the tip of the polymer remains
in the _ form
T form
tip of the polymer remains
in the T form forms a
GTP cap
an end might grow for a certain length of time in a _ form, but then suddenly change to the _ form and
begin to shrink rapidly
T
D
rapid interconversion between a growing
and shrinking state
dynamic instability
growth to shrinkage
catastrophe
shrinkage to growth
rescue
produce straight
protofilaments that make strong and regular lateral contacts with one another
tubulins subunits with GTP bound to the
β-monomer
tubulins subunits with GTP bound to the β-monomer produce?
straight protofilaments
associated with subtle conformational change in
the protein
hydrolysis of GTP to GDP
subtle conformational change in the protein, makes the protofilaments?
curved
constrain the curvature of the protofilaments, the ends appear straight
GTP cap
terminal subunits have hydrolyzed
constrains is removed, spring apart
2 polymer drugs that inhibit the microtubule functions
polymer-stabilizing and polymer destabilizing drugs
interact with tubulin subunits and lead to
microtubule depolymerization
colchicine and nocodazole
binds to and stabilized
microtubules
Taxol
Taxol binds to and stabilizes microtubules, causing a net increase in
tubulin polymerization
used to treat cancers of
the breast and lungs
Taxol
Microtubule drugs that preferentially kill dividing cells
microtubule-depolymerizing and
polymerizing drugs
______ ___ _____ _____ required for spontaneous nucleation of
microtubules is very high
concentration of tubulin subunits
smaller amounts, involved in the nucleation of microtubule growth
γ-tubulin
Microtubules are generally
nucleated from a specific intracellular location known as a
microtubule-organizing center (MTOC)
two accessory proteins bind directly to the γ-tubulin, along with several other proteins
that help create a spiral ring of γ-tubulin
molecules, which serves as a template
that creates a microtubule with 13
protofilaments
γ-tubulin ring complex
two accessory proteins bind directly to the
γ-tubulin, along with several other proteins
accessory proteins bind directly to the γ-tubulin, along with several other proteins that help create a
spiral ring of γ-tubulin
molecules
spiral ring of γ-tubulin
molecules, which serves as a
template
that creates a microtubule with 13
protofilaments
well-defined MTOC, which is located near the nucleus
centrosome
well-defined MTOC called the centrosome,
which is located near the nucleus and from which ______are nucleated at their minus ends, so the plus ends point outward and continuously grow and
shrink
microtubules
embedded in the centrosome; a pair
of cylindrical structures arranged at right angles
in an L-shaped configuration; barrel shape with
striking ninefold symmetry
centrioles
centrioles, a pair of cylindrical structures arranged at
right angles
in an L-shaped configuration
centrioles are embedded in the
centrosome
where microtubule nucleation takes place
pericentriolar material
MTOC embedded in
the nuclear envelope found in budding
yeast, fungi, and diatoms
spindle pole body
no centrioles in
fungi or plants
all these cells use this to nucleate their microtubules
γ-tubulin
with dynamic plus ends pointing outward toward the cell periphery and stable minus ends collected near the nucleus
aster-like configuration
aster-like configuration of microtubules is robust,
with dynamic plus ends pointing
outward toward the cell periphery and stable
minus ends collected near the nucleus
has the ability
to find the center of the cell established
a general coordinate systems
microtubule cytoskeleton
microtubule cytoskeleton has the ability
to find the center of the cell established
a general coordinate systems, which is then used to
position many organelles within the cell
exhibit much higher polymerization rate, a greater catastrophe frequency, and extended pauses in microtubule
growth
microtubules in cell
microtubules in cell exhibit?
- much higher polymerization rate,
- a greater catastrophe frequency, and
- extended pauses in microtubule growth
modulate filament dynamics and
organization
microtubule-binding proteins
proteins that bind to microtubules
microtubule-associated proteins (MAPs)
can stabilize microtubules against disassembly
microtubule-associated proteins (MAPs)
mediate interactions with other cell components – prominent in neurons,
axons and dendrites that extend from the
cell body
MAPs
MAPs are prominent in
neurons, axons and dendrites that extend from the cell body
MAPs have at least __ ___that binds to the
microtubule surface and another that projects outward.
one domain
long projecting domain; form bundles of stable microtubules that are
widely spaced
MAP2
shorter projecting domain, form bundles of more closely packed
microtubules
tau
are targets of several protein kinases
MAPs
influence stability and dynamics
proteins that bind the ends of microtubule
rate at which a microtubule switches from growing to a shrinking state
frequency of catastrophe
rate at which a microtubule switches from shrinking to
growing state
frequency of rescues
bind to
microtubule ends and appear to pry
protofilaments apart
catastrophe factors (kinesin-13)
protects microtubule
minus ends from the effects of
catastrophe factors
Nezha / Patronin
enriched at microtubule plus
ends; binds free tubulin subunits and
delivers them to the plus end; promoting
microtubule polymerization and
simultaneously counteracting catastrophe
factor activity
XMAP215
stabilized by association with
a capping protein or the centrosome;
depolymerization sites
minus ends
explore and probe the entire
cell space
plus ends
accumulate at these active ends (+) and
appear to rocket around the cells as
passengers at the ends of rapidly growing
microtubules; dissociating from the ends
when microtubules shrink
plus-end tracking proteins (+TIPs)
behave as +TIPs and act to modulate the growth and
shrinkage of microtubule
kinesin-related catastrophe factors and
XMAP215
control microtubule positionin
kinesin-related catastrophe factors and
XMAP215
(Other +TIPs)
small dimeric proteins; attach to the
plus end; allow the cell to harness the
energy of polymerization; used for
positioning the spindle, chromosomes, or
organelles
EB1
unpolymerized tubulin subunits to
maintain a pool of active subunits
cell sequester
binds to two tubulin
heterodimers and prevents their addition to the
ends of microtubules; decrease the effective
concentration of tubulin subunits
stathmin (Op18)
inhibits its binding to
tubulin
stathmin phosphorylation
“sword”; made up for two subunits,
smaller ones hydrolyze ATP performs the actual
severing, larger on directs katanin to the
centrosome
katanin
microtubules also use _____ proteins to
transport cargo and perform a variety of
other functions
motor proteins
two types of motor proteins
kinesins and dyneins
kinesin-1 is alsi called
conventional kinesin
carriers membrane-enclosed organelles away from the cell body toward the axon terminal by walking toward the plus end of microtubule
kinesin-1 (“conventional kinesin”)
Kinesin-1 is similar to
myosin II in having two heavy chains per active motor
is the common element of myosin and kinesin
motor domain
How many distinct families in kinesin superfamily
14
Most of fourteen distinct families in the kinesin superfamily have the?
motor domain at the N-terminus of the heavy chain and walk toward the plus end of the microtubule.
uses the ATP hydrolysis to depolymerize microtubule ends
motor domain
has a central motor domain and does not walk at all, but uses the
energy of ATP hydrolysis to depolymerize microtubule ends
kinesin-13
have a binding site in the tail for another microtubule
Most kinesins
instead of the rocking of a lever arm, small movements at the nucleotide-binding site regulate the docking and undocking of the motor head domain to a long linker region.
kinesin-1
docking and undocking of the motor head
domain to a long linker region acts to throw the
second head forward to a binding site 8 nm closer to the microtubule plus end,
closely coordinated, so that this cycle of linker
docking and undocking allows the two-headed motor to move in a hand-over-hand (or head-over-head) stepwise manner
nucleotide-hydrolysis cycles
allows the two-headed motor to move in a hand-over-hand stepwise manner
cycle of linker docking and undocking
family of minus-end directed microtubule motors unrelated to the kinesins
dyneins
dyneins are composed of
one, two, or three heavy chains (that include
the motor domain) and a large and variable number of associated intermediate,
light-intermediate, and light chains
two major branches of dynein family
cytoplasmic dyneins (first branch)
Axonemal dyneins (second branch)
homodimers of two heavy chains
cytoplasmic dyneins
used for organelle and mRNA trafficking, for positioning the centrosome and nucleus during cell migration, and for construction of the microtubule
spindle
cytoplasmic dynein I
have cilia and is used to transport material from the tip to the base
of the cilia
Cytoplasmic dynein 2
highly specialized for the rapid and efficient sliding movements of microtubules that drive the beating of cilia and flagella
axonemal dyneins (ciliary dyneins)
the largest of the known molecular motors,
Dyneins
follows the general rule of coupling nucleotide
hydrolysis to microtubule binding and unbinding as well as to a force-generating
conformational change
dynein motor
major function of cytoskeletal motors in interphase cells
transport and positioning of membrane-enclosed organelles
Kinesin was originally identified as the
protein responsible for fast anterograde axonal transport
movements toward the cell’s periphery
antegrade axonal transport
identified as the motor responsible for
transport in the opposite direction, retrograde axonal transport
Cytoplasmic dynein
movement towards the cell center
retrograde axonal transport
require the action of minus-end directed cytoplasmic dynein
centripetal movements toward the cell center
centripetal movements toward the cell center require the action of
minus-end directed cytoplasmic dynein
