Test 3. Lecture 28 Flashcards
- Myosin heads bind and hydrolyze ________, providing energy to drive
filament sliding. - Myosin changes shape during repeated cycles of interaction between
myosin heads and ________.
The conformational changes in myosin result in movement of myosin
heads along actin filaments. - The model of myosin function comes from in vitro studies and
determination of the 3-D structure of ________:
• Binding of ATP dissociates myosin from actin.
• ATP hydrolysis induces a conformational change that displaces the
myosin head group.
- ATP
- actin
- myosin
• The myosin head binds to a new position on the actin filament and Pi
is
released.
• The “_____________”: myosin head returns to its original conformation,
which drives actin filament sliding, and ADP is released.
power stroke
Contraction of ____________ is
triggered by nerve impulses which
stimulate release of Ca2+ from ____________
skeletal muscle
sarcoplasmic reticulum
Increased Ca2+ concentration in the
cytosol affects two actin filament
binding proteins: _______ and
__________
tropomyosin
troponin
Tropomyosin binds \_\_\_\_\_\_\_\_\_\_ along actin filaments, and is also bound to troponin. When Ca2+ is absent, the tropomyosintroponin complex blocks binding of myosin to actin. Binding of Ca2+ to troponin C shifts complex, and allows contraction to proceed.
lengthwise
In ___________, contractile assemblies are similar to muscle fibers.
They also produce contraction by sliding of actin filaments relative to one
another.
Stress fibers and adhesion belts are examples of contractile assemblies
nonmuscle cells
__________: division of a cell
following mitosis.
A \_\_\_\_\_\_\_\_\_\_ of actin and myosin II is assembled by membrane-bound myosin just beneath the plasma membrane. Contraction of the ring pinches the cell in two.
Cytokinesis
contractile ring
In ____________ and smooth muscle, contraction is regulated primarily
by _______________ of a myosin light chain.
It is________________, which is regulated by
the Ca2+-binding protein ____________.
nonmuscle cells
phosphorylation
myosin light chain
kinase (MLCK
calmodulin
“Unconventional myosins:”
_____________: don’t form filaments and are not involved in contraction.
They function in a variety of cell movements, such as transport of vesicles and
organelles.
Nonmuscle myosins
___________: 1. Have globular head groups that act as molecular motors.
2. Short tails bind to other structures. Movement of myosin I along an actin
filament can transport its attached cargo, such as a vesicle.
Myosin I
\_\_\_\_\_\_\_\_\_\_\_\_\_: Two-headed dimer that transports vesicles and other cargo along actin filaments. Some unconventional actins are involved in actin filament reorganization and anchor actin filaments to the plasma membrane.
Myosin V
__________are rigid hollow rods.
- They are dynamic structures that undergo continual
assembly and disassembly. - They function in cell movements and determining cell shape.
- Microtubules are made of the globular protein ___________.
Tubulin dimers consist of ____________, which are
encoded by related genes.
γ-tubulin in the centrosome helps in initiating microtubule
assembly.
Microtubules
tubulin
α-tubulin and β-tubulin
Tubulin dimers polymerize to form
microtubules: 13 protofilaments
around a hollow core.
Protofilaments are head-to-tail arrays of
_____________ arranged in parallel.
Microtubules have polarity (plus and
minus ends), which determines
direction of movement.
tubulin dimers
Microtubules can undergo rapid
cycles of assembly and
disassembly.
____________is hydrolyzed
to GDP SHORTLY AFTER POLYMERIZATION
This weakens binding affinity of tubulin dimers for each other, causing rapid depolymerization and loss of tubulin bound to GDP from the minus end.
GTP bound to β-tubulin
In microtubules stabilized at the minus end, rapid GTP hydrolysis results in dynamic
___________: alternating between cycles of growth and shrinkage.
As long as new GTP-bound tubulin dimers are added more rapidly than GTP is
hydrolyzed, a GTP cap remains at the plus end microtubule growth continues.
If GTP is hydrolyzed more rapidly than new subunits are added, GDP-bound tubulin
at the plus end of the microtubule leads to disassembly and shrinkage.
instability
Rapid turnover of microtubules allows for remodeling of the
cytoskeleton that occurs during mitosis.
Drugs such as __________ and ___________ that affect
microtubule assembly are useful as experimental tools and in
cancer treatments.
colchicine
colcemid
__________ and _____________ are used in cancer
chemotherapy because they INHIBIT MICROTUBLE POLYMERIZATION
and thus affect rapidly dividing cells.
Vincristine
vinblastine
_______STABILIZES MICROTUBULES, which also blocks cell division.
Taxol
______________regulate the dynamic behavior
of microtubules.
Minus ends are stabilized by proteins that prevent depolymerization.
Growth or shrinkage of plus ends is regulated by MAPs.
Microtubule-associated proteins (MAPs)
In animal cells, most microtubules
extend outward from the
_____________
During mitosis, they extend outward
from duplicated centrosomes to
form the mitotic spindle.
The spindle is responsible for
separation and distribution of
chromosomes to daughter cells..
centrosome
Centrosomes are initiation sites for microtubule assembly, which grow outward _________anchored in the centrosome
If cells are treated with __________, microtubules disassemble. When the drug is
removed, new microtubules can be seen growing outward from the centrosome.
from
minus ends
colcemid
γ-tubulin in the centrosome is associated with other proteins in a ring-shaped
structure called the ____________
This complex is thought to bypass the rate-limiting nucleation step.
g-tubulin ring complex.
Most animal cell centrosomes have a __________ oriented PERPENDICULAR TO EACH OTHER
and surrounded by _____________-
pair of centrioles,
pericentriolar material`
___________ are cylindrical, based on ____________
Centrioles also form ___________________
But they are not found in plant cells, many unicellular eukaryotes, and most meiotic
animal cells.
In these cells the pericentriolar material initiates microtubule assembly
Centrioles
nine triplets of microtubules
basal bodies of cilia and flagella.
Microtubule stability is also regulated by post-translational
modification of tubulin by phosphorylation, acetylation, etc.
These modifications affect microtubule behavior by providing
sites for binding of specific MAPs.
Interactions with MAPs allow cells to __________________in
particular locations and help determine cell shape and
polarity.
Many MAPs are cell-type specific.
The tau protein is a MAP and is characteristic of lesions found in
the brains of Alzheimer’s patients
stabilize microtubules
Nerve cells have two types of processes supported by stable microtubules.
____________: microtubules have plus ends towards the tips; associated with
tau.
______________: microtubules are oriented in both directions; associated with
MAP2.
axons
Dendrites
Two families of motor proteins are responsible for powering movements
in which microtubules participate.
_________: most move along microtubules toward the PLUS END.
__________: move toward the MINUS END.
Kinesins
Dyneins
A major role of microtubules is to transport vesicles and organelles through
the cytoplasm.
Different members of the __________ and ____________families are thought to
transport cargo in opposite directions.
kinesin
dynein
