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Cell Biology > Lecture 20: Cytoskeleton > Flashcards

Lecture 20: Cytoskeleton Flashcards

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1
Q

cytoskeleton

A

is a series of long, filamentous protein fibres that are formed within the cell.

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2
Q

Intermediate filaments

A

tough, ropelike fibers
- made of a variety of
related proteins

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3
Q

Microtubules

A
  • hollow, rigid
    cylindrical tubes
  • made from tubulin
    subunits
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4
Q

Microfilaments

A

solid, thinner structures

- made of actin

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5
Q

Describe the various functions of the cytoskeleton

A

provides structural support

intracellular transport of organelles and vesicles

as a force generating apparatus

provides an internal framework

essential component of the apparatus that
divides chromosomes during mitosis and meiosis

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6
Q

Describe how the cytoskeleton provides structural support

A

The cytoskeleton provides structural support that can determine the shape of the cell and resist forces that can deform the cell

• A cell’s shape is closely
associated with its function
• Cytoskeleton can resist
mechanical stresses
• Cytoskeleton can rearrange
itself to change the shape
of cells
All the cell-cell
connections are
connected to the
inside of the cell via
the cytoskeleton,
providing support in
cells and tissues
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7
Q

Describe how the cytoskeleton functions in

intracellular transport

A

The intracellular transport of organelles and vesicles occurs via the cytoskeleton

• Machinery needed to move materials &
organelles within cell; network of highways
directs movement

• Movement of mRNA molecules to specific
parts of cell

• Movement of vesicles from ER to Golgi
complex

• Movement of neurotransmitter-containing
vesicles from synthesis site to axon terminal

• Transport of peroxisomes over rails of
microtubules; the two are closely associated
(microtubules in red, peroxisomes in green)

Some tracts are really long… some neurons
extend from the spinal cord to the end of your
fingers! Vesicles are carried along microtubules
in the axon. It can take weeks for transport!

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8
Q

Describe how the cytoskeleton functions as a a force generating apparatus

A

Cytoskeletal elements can function as a force generating apparatus
that moves cells from one place to another (if they’re mobile!)

Single-celled organisms move by crawling
over surface or propelled by protruding
cilia & flagella

Multicellular organisms have variety of
cells capable of independent locomotion - sperm, white blood cells, fibroblasts,
highly motile tip of growing axon
(movement like crawling white blood cell)

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9
Q

Describe how the cytoskeleton provides an internal framework

A

The cytoskeleton provides an internal framework for positioning the
various organelles within the interior of the cell.

Intracellular organization is
disrupted by drugs or mutations
that interfere with normal
cytoskeleton structures

The location of cellular components isn’t
random… they’re all anchored to and
moved around the cytoplasm by the
cytoskeleton

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10
Q

Describe what role the cytoskeleton plays in mitosis/ meiosis

A

is an essential component of the apparatus that
divides chromosomes during mitosis and meiosis
the spindle apparatus made of
microtubules

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11
Q

Describe microtubule structure

a. Explain why microtubules have polarity

A

• Basic unit is a tubulin dimer (microtubule subunit) composed of
alpha (α) and beta (β) tubulin subunits (heterodimer)

polymerize to form a cylinder… the dimers are
not covalently attached to each other

microtubules are therefore
asymmetric & polarized (β end is plus; α end is
minus)

The plus ends have higher rates of tubulin
addition (thus grow faster); the minus ends
are slower growing

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12
Q

What is a protofilament?

A

• One ‘strand’ of the tube is called a
protofilament

all protofilaments in single MT have same
polarity - microtubules are therefore
asymmetric & polarized (β end is plus; α end is
minus)

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13
Q

β -tubulin can have a _________or

_____ bound

A

β -tubulin can have a swappable GDP or

GTP bound

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14
Q

α-tubulin ____

has____ bound

A

a- tublin permanenlty has a GTP bound

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15
Q

Describe how microtubules assemble

A

• initial polymerization is slow – requires
nucleation (coming together to build starting point) of 13 dimers, with the help of
another tubulin: γ (gamma) tubulin and
other proteins

• γ tubulin dissociates from microtubule
after polymerization has progressed

  • subsequent subunit addition is faster (as dimers keep getting added affinity increases
  • depolymerization also occurs spontaneously, but at a slower rate.

microtubules are in constant flux,
polymerizing and depolymerizing
• one end of the MT is slower growing (-) while the other is faster (+) giving each microtubule a polarity

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16
Q

What determines rate of assembly and disassembly?

A

Microtubules may vary in their rate of assembly and disassembly
Tubulin half life is nearly a full day, however, the half life of a given microtubule may be only 10 minutes.

- rate of assembly is dependent on how many tubulin dimers are floating around freely in the cytoplasm
• relative rates of assembly and
disassembly determine their growth
– dependent on local tubulin
concentration
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17
Q

γ (gamma) tubulin

A

dissociates from microtubule
after polymerization has progressed

helps with initial polymerization

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18
Q

What controls microtubule instability?

A

microtubule’s instability is controlled by hydrolysis of GTP bound by tubulin.

  • tubulin dimers bind GTP before binding microtubule
  • the microtubule dimer hydrolyzes the GTP to GDP.
  • this reduces the affinity of the tubulin for its neighbor, thus leading to disassembly. (destabilize)

• To retain some stability, the GDP- dimers can be regularly replaced
with more GTP-dimers

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19
Q

What other thing also affects disassembly rates?

A

Proteins called +TIPs can also affect disassembly rates.

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20
Q

List Factors that affecting microtubule stability

A
  1. Rate of growth: more growth stabilizes the microtubule because added tubulin dimers have GTP bound (forming a
    GTP cap)
    • The rate of growth is largely dependent on the concentration of tubulin
    dimers available in the vicinity of the growing end
  2. Rate of GDP to GTP exchange: as the dimers hydrolyze GTP further down the MT (thus destabilizing the microtubule), new GTP can be enzymatically swapped in to stabilize the microtubule
  3. Association with microtubule associated proteins,
    some of which help stabilize the microtubule
21
Q

Describe how chemotherapy drugs target microtubules

A

• cancer cells, with their high rate of
division, require faster cycling of tubulin
– drugs can prevent that

• Drugs that interfere with MT dynamics
will interfere with the spindle apparatus
that divides chromosomes in mitosis

• Normal cells have a ‘mitotic checkpoint’
that will stop mitosis if something is
wrong. Cancer cells often lack these
checkpoints… thus will try to divide even
with a ‘poisoned’ spindle. This causes
uneven distribution of chromosomes and
will often kill the cancer cell.

22
Q

List drugs which affect assmbly/disassembly

A
  • colchicine, nocodazole make microtubules
    disassemble
  • Taxol prevents disassembly
  • Vinblastine prevents assembly
23
Q

MTOCs; Microtubule organizing centers

A

initiate microtubule polymerization

The best understood is are centrosomes and basal bodies.

These are actually interchangeable… the basal body from a sperm becomes the
centrosome of the newly fertilized egg!

24
Q

Describe centrosome structure

A

is comprised of two centrioles surrounded by a cloud of molecules
known as the pericentriolar material (or centrosome matrix)

• In centrosome, centrioles usually in pairs, perpendicular to each other

25
Q

Centrosomes

A

are usually close to
the nucleus, and microtubules
extend from them.

26
Q

Basal bodies

A

assemble the microtubules that make up cilia and

flagella.

27
Q

What is an example of MTOC in animal cells?

A

centrosome

28
Q

• Centrioles

A

are cylindrical structures 0.2 um in diameter and about 0.4 um long

are not actually involved in microtubule nucleation, but “recruit” the
molecules that do (gamma tubulin and other proteins)

29
Q

Do plants have centrosomes?

A

plants lack centrosomes – their MTOCs are typically embedded in the nuclear membrane

30
Q

. List the functions of microtubule associated proteins

A

• stabilize microtubules
• alter assembly/disassembly
rates
• crosslink adjacent MTs

MAP activity controlled by addition
& removal of phosphate groups on
particular amino acid residues by
protein kinases & phosphatases,
respectively
31
Q

How may maps be associated with Alzheimer’s disease?

A

• Abnormally high phosphorylation of a
MAP is implicated in fatal
neurodegenerative diseases like AD

• hyperphosphorylated tau protein sticks
together into neurofibrillary tangles in
neurons; causes microtubules to
disintegrate;

• short microtubules prevent normal
intracellular transport and therefore kill
nerve cells, resulting in brain
deterioration

This happens in
addition to the
amyloid plaques that
are misfolded. So lots
can get messed up in
neurons in
Alzheimer’s patients.
32
Q

What are some possible AD therapies?

A

specific kinase

inhibitors

33
Q

Motor proteins

A

convert chemical energy stored in ATP into mechanical energy that is used to generate force or to move cellular cargo attached to the motor

B. A single cell may contain dozens of different motor proteins, each specialized
for moving a particular type of cargo in a particular cell region

34
Q
  1. Describe the three families of motor proteins
A
  1. Kinesins move toward (+) end of
    microtubules
  2. Dyneins move toward (-) end of MTs

They can attach to different adapter proteins
that then attach to proteins imbedded in
vesicle membranes (Rabs), the cell membrane,
or adjacent microtubules to generate
movement

Microfilament motor proteins:
3. Myosins

(There are no known motor proteins that
use intermediate filaments as tracts)

35
Q

Types of cellular cargo transported by these molecular motors include:

A

ribonucleoprotein particles, vesicles, organelles (mitochondria, lysosomes,
chloroplasts), chromosomes & other cytoskeletal filaments

36
Q

i

A

Motion is a bit like rope-climbing or swimming: alternate between power strokes and
recovery strokes:

37
Q
A

Note that the ‘cargo’ can have different

motor proteins attached to them at once

38
Q

Kinesin

A

move cargo to the + end of a microtubule

are tetramer proteins constructed of 2 identical heavy chains & 2 identical light chains

They`re the smallest of motor proteins (but still big!)

• There are approx 45 different kinesins in humans, each binds a
different subset of cargo

39
Q

Describe Kinesin structure

A

A pair of globular heads that bind a MT & generate force by hydrolyzing ATP (thus called the motor domain)

Each head is connected to a neck

The rest of the protein is a rod-like
stalk & a fan-shaped tail that binds
cargo to be hauled

40
Q

How does Kinesin move?

A

• Kinesin movements mediated by ATP hydrolysis : 1 ATP per
“step”

• Moves along single MT protofilament (rate proportional to
[ATP]; up to 800 nm/sec)

• Move in distinct steps (1 dimer at time; 8 nm); usually toward plasma membrane (e.g from Golgi to plasma membrane)

attaches to one of the vesicle’s integral proteins (often via Rab proteins; not
shown in this figure).

Movement is prossessive because at least one head is attached to the microtubule at all times. It thus can walk and carry cargo micrometers down a MT

41
Q

Describe how kinesin hydrolyse ATP to walk along MTs

A

before attaching to a MT, both heads have ADP bound)

  1. The front head binds to a microtubule and
    releases it’s ADP.
  2. ATP binds to the front motor domain and
    causes a conformational change (the neck
    region associates with the head region). This
    causes the back head to swing forward and
    bind the MT.
  3. ADP is released from the (now) forward head.
    The (now) back head hydrolyses the ATP to ADP.
    The energy released dissociates the head from
    the microtubule.
42
Q

How does kinesin function in neuronal transport?

A

• Kinesin I was first isolated in 1985
from squid giant axons

• It is responsible for axonal flow, in
which vesicles (e.g., precursors of
synaptic vesicles formed in the
Golgi) are carried along
microtubules from the cell body to
axon endings.
• Axon endings have an extensive
actin cytoskeleton. Myosin V may
take over to transport vesicles
along actin filaments to near the
plasma membrane at the synapse.
43
Q

Cytoplasmic dyneins

A

carry cargo to – MT ends

ubiquitous eukaryotic motor protein, only 2 versions in humans

  • huge protein (1500 kDa)
  • two identical heavy chains: head generates force

• variety of intermediate & light chains (each with different cargo
recognition domains)

• cargo attached by dynactin

44
Q

Describe the various dynein functions

A
  • chromosome movement in mitosis
  • positioning Golgi complex

• movement of vesicles & organelles
through cytoplasm

• involved in axonal retrograde organelle
movement

• in fibroblasts & endothelial cells move
endosomes, lysosomes, Golgi-derived vesicles toward cell center

• also ‘carries’ HIV to the nucleus

45
Q

Endothelium cells

A

line the interior surface of blood vessels and lymphatic vessels

46
Q

Fibroblast

A

a cell in connective tissue that produces collagen and other fibers

47
Q

Does GTP or GDP stabilize the MT?

A

GTP stabilizes GDP detsabilizes

48
Q

Compare a growing Mt vs a shrinking MT

A

Growing;
GTP - tubulin dimers add to growing end of MT
addition proceeds faster than GTP hydrolysis by dimer

Shrinking

  • GTP hydrolysis is faster than addition of new GTP bound tubulin dimers (GTP cap is lost)
  • portofilaments containing GDP peel away
  • GDP- tubulin gets released into cytosol

Cell Biology (23 decks)

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