Test 3. Lecture 28

Question 1

1. Myosin heads bind and hydrolyze ________, providing energy to drive
   filament sliding.
2. 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.
3. 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.

Answer 1

1. ATP
2. actin
3. myosin

Question 2

• 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.

Answer 2

power stroke

Question 3

Contraction of ____________ is
triggered by nerve impulses which
stimulate release of Ca2+ from ____________

Answer 3

skeletal muscle

sarcoplasmic reticulum

Question 4

Increased Ca2+ concentration in the
cytosol affects two actin filament
binding proteins: _______ and
__________

Answer 4

tropomyosin

troponin

Question 5

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.

Answer 5

lengthwise

Question 6

 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

Answer 6

nonmuscle cells

Question 7

__________: 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.

Answer 7

Cytokinesis

contractile ring

Question 8

In ____________ and smooth muscle, contraction is regulated primarily
by _______________ of a myosin light chain.

It is________________, which is regulated by
the Ca2+-binding protein ____________.

Answer 8

nonmuscle cells
phosphorylation

myosin light chain
kinase (MLCK
calmodulin

Question 9

“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.

Answer 9

Nonmuscle myosins

Question 10

___________: 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.

Answer 10

Myosin I

Question 11

\_\_\_\_\_\_\_\_\_\_\_\_\_: 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.

Answer 11

Myosin V

Question 12

__________are rigid hollow rods.

1. They are dynamic structures that undergo continual
   assembly and disassembly.
2. They function in cell movements and determining cell shape.
3. 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.

Answer 12

Microtubules

tubulin

α-tubulin and β-tubulin

Question 13

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.

Answer 13

tubulin dimers

Question 14

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.

Answer 14

GTP bound to β-tubulin

Question 15

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.

Answer 15

instability

Question 16

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.

Answer 16

colchicine

colcemid

Question 17

__________ and _____________ are used in cancer
chemotherapy because they INHIBIT MICROTUBLE POLYMERIZATION
and thus affect rapidly dividing cells.

Answer 17

Vincristine

vinblastine

Question 18

_______STABILIZES MICROTUBULES, which also blocks cell division.

Answer 18

Taxol

Question 19

______________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.

Answer 19

Microtubule-associated proteins (MAPs)

Question 20

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..

Answer 20

centrosome

Question 21

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.

Answer 21

from
minus ends

colcemid

Question 22

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.

Answer 22

g-tubulin ring complex.

Question 23

Most animal cell centrosomes have a __________ oriented PERPENDICULAR TO EACH OTHER
and surrounded by _____________-

Answer 23

pair of centrioles,

pericentriolar material`

Question 24

___________ 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

Answer 24

Centrioles
nine triplets of microtubules

basal bodies of cilia and flagella.

Question 25

 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

Answer 25

stabilize microtubules

Question 26

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.

Answer 26

axons

Dendrites

Question 27

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.

Answer 27

Kinesins

Dyneins

Question 28

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.

Answer 28

kinesin

dynein