Lecture 21: Cytoskeleton II

Question 1

List the basic properties of cilia and flagella

Answer 1

• Both are hair-like structures that
project from cell surface
• both are used for locomotion
• distinction between cilia and flagella is
blurred, because both are made of the
same components

Question 2

Compare and contrast cilia and flagella

Answer 2

cilia are typically shorter and more numerous

• cilia are like oars – cell movement is perpendicular to direction of cilia themselves
• flagella are longer, rarely more than two
• flagella tend to be aligned in the direction of movement

Cilia move single celled organisms, or material around in multicellular organisms. These are lung epithelial cells.

Question 3

Describe the structure of cilia and flagella

Answer 3

• both are made of microtubules surrounded by plasma membrane
• core is 9 + 2 array of MTs called the axoneme; misnomer
• 9 peripheral doublet MTs with 2 central single MTs; 9 x 1.8 x2 (there’s not complete 2 MT in each doublet)
• all MTs have + ends at tip, - ends at base
• At the base of the flagellum, there is a MTOC called the basal body (looks exactly like a centriole)

Question 4

Describe the structure of the axoneme

Answer 4

• each MT doublet in the axoneme
has:
• 1 complete MT (A -13
protofilaments) and
• 1 incomplete MT (B- with 10
or 11 protofilaments)- clamps on to the side of the complete MT 

• A-tubule is linked to B-tubule of
adjacent doublet by nexin

• ciliary dynein arms and radial
spokes are attached to A tubules

Question 5

ciliary dynein

Answer 5

arms and radial

spokes are attached to A tubules

Question 6

nexin,

Answer 6

s an elastic protein

binds doublet together but connects each doublet with one beside it

Question 7

axoneme

Answer 7

core is 9 + 2 array of MTs
• 9 peripheral doublet MTs with 2
central single MTs

Question 8

Describe how dynein and nexin move the axoneme

Answer 8

MT doublets slide past each other. Dynein arms ‘walk’ along
tubulin wall of the adjacent doublet.
1. Dyneins attach one microtubule as ‘cargo’
2. ATP hydrolysis allows dynein to ‘walk’ along neighboring microtubule
3. Nexin proteins linking the two doublets create a bend
4. Dynein releases
5. The cycle repeats

Question 9

. Describe the hypothesized mechanism in which the “central pair” control the beating of the
axoneme

Answer 9

Hypothesis: activity of dynein arms is
regulated by central MT pair & radial spokes
• central pair rotates as beating proceeds
• rotation promotes sequential contacts with
each radial spoke, making the spoke signal
to the dynein arm on the adjacent A MT

Answer 10

Alternation of this sliding causes the cilia or
flagella to bend
The sliding on one side of axoneme alternates
with sliding on other
• cilium first bends one way, then other
• activity of dynein arms alternates from one
side to other

Question 11

List the properties and functions of primary cilia

Answer 11

Primary cilia are on some cells and differ
from other cilia in that they lack the
central pair of microtubules; ( 9+0)

• They do not have a locomotory function
• There is only one per cell

• They are a type of antenna, picking up
chemical signals (they are covered in cell
surface receptors), and they carry other
signaling molecules up and down the
microtubules, into the cell

• They can act as mechanoreceptors
(bending send signal), sensing flow outside
cell

Question 12

What does primary cilia not having a locomotion function suggest?

Answer 12

suggests the central pair of MT are key to locomotion

ex- of reverse genetics (if you delete something and then look at its funtcion and see how imp it is)

Question 12

What does primary cilia not having a locomotion function suggest?

Answer 12

suggests the central pair of MT are key to locomotion

ex- of reverse genetics (if you delete something and then look at its funtcion and see how imp it is)

Question 13

List the properties of intermediate filaments

Answer 13

Intermediate in size (10 nm
diameter) between MT (25 nm)
and microfilaments (8 nm)

Strongest tensile strength of all
cytoskeleton fibers (some have been found to tolerate being stretched to 3x normal length)

Made of fibrous proteins that intertwine with each other to make dense, rope-like fibers

Especially abundant in cells that endure physical stresses such as muscle cells, neurons, and epithelial cells

Connected to other components of cytoskeleton by cross-linking proteins, such as plectin

Question 14

In which types of organisms have IF been found?

Answer 14

intermediate filaments have only
been observed in animal cells. All other
lineages, including prokaryotes, have MTlike and actin-like proteins.

Question 15

Do IF have polarity?

Answer 15

Note: unlike MTs and microfilaments, these have no
polarity and there are no motor proteins that use
them for tracts.

IF proteins are a heterogeneous group

The primary amino acid sequence
of these can be quite different. The
all however exhibit similar
structural properties.

Question 16

Describe how intermediate filaments assemble

Answer 16

Despite differences in type of protein, all IFs share
similar structure

Each has conserved central, α-helical domain

Differences arise due to globular domains of variable size & sequence that flank the helical core.

Two monomers wrap around each other in coiled-coils to form dimers with both subunits aligned parallel to each other

Dimers bind to each other to form tetramers. Dimers
are aligned in opposite orientations & are staggered: tetramers are NOT POLAR (no plus or minus end)

Tetramers aggregate in a staggered manner to form 60 nm filament, which aggregates to form IFs.
IFs continuously assemble and disassemble (like MTs), but subunits can insert in the middle, not the ends of the filaments. No ATP or GTP needed.

Assembly/disassembly controlled by phosphorylation
(e.g. MPF phosphorylates lamins in mitosis)

Question 17

. List the functions of intermediate filaments

Answer 17

Main function is providing mechanical
support because they are highly
resistant to tensile forces

Hair, fingernails are made of keratin IFs
skin is mainly a mat of keratin Ifs

Nuclear lamins form network lining
inside of nuclear envelope.

Neurons are filled with neurofilaments

IFs are useful in cancer diagnosis

Question 18

. How may intermediate filaments identify the origins of metastatic cancer cells?

Answer 18

Many cell types have specific IF profile.
Keratins have subtypes that are unique to
different epithelial cells (bladder, skin, etc)
or even different subsets of one cell type
(like basal epidermal cells). This is useful in
detection of the origin of cells in a tumor,
especially cells that have metastasized

Question 19

Explain how Intermediate filaments help hold cells together

Answer 19

In epithelia, keratin
intermediate filaments form
junctions that hold cells
together (desmosomes), or
attach cells to matrix
(hemidesmosomes).

The intermediate filaments
loop into the plaques and
spread out into the
cytoplasm. This links two
cells together structurally

Question 20

Epithelial tissues

Answer 20

line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs.

Question 21

List the functions of actin filaments

Answer 21

forms a band just beneath the plasma membrane
that provides mechanical strength to the cell

• links transmembrane proteins (e.g. cell surface
receptors) to cytoplasmic proteins

• anchors the centrosomes at opposite poles of the
cell during mitosis pinches dividing animal cells
apart during cytokinesis

• generate locomotion in cells such as white blood
cells and the amoeba

• interact with myosin filaments in skeletal muscle
fibers to provide the force of muscular contraction

Question 22

microfilaments

Answer 22

Thinnest of the 3 cytoskeletal fibers (8 nm)

Composed of actin -

Question 23

actin

Answer 23

monomers of the protein actin polymerize to form long, thin
and flexible fibers

Actin needs ATP to become part of the
growing filament; ATP is hydrolyzed once
actin is part of the microfilament

Answer 24

Each actin subunit is
made of 4 subdomains
“pointed” end (-)
“barbed” end (+)

Note that all of the
actin subunits are
made of the same
actin (i.e. it’s a
homopolymer).

Question 25

How is the polarity of actin filaments defined?

Answer 25

Because of the shape of actin subunits, one end of a
micro-filament appears pointed, and the other
appears barbed.
– Pointed end = minus end
– Barbed end = plus end

Question 26

What is actin assembly/disassembly dependent on?

Answer 26

Actin assembly/disassembly in vitro depends upon concentration of actin monomers. Filament
assembly leads to drop in ATP-actin. Actin subunits are then preferentially added to plus end
and removed from the minus end (steady state). Microfilament cytoskeleton is organized by
controlling equilibrium between assembly and disassembly of microfilaments.

Question 27

Describe how microfilaments assemble

Answer 27

Step 1: Add a preformed actin filament to a
solution of actin subunits and ATP.
Yellow beads = filament
Red beads = actin subunits added

Step 2: increased concentration of ATP-actin,
actin is added at both ends.

Step 3: Drop in ATP-actin, addition of
monomers at plus end (higher affinity for ATPactin).

Step 4: subunits continually added to plus
end, and now being lost from minus end.

Step 5: + end still gains and – end loses ATPactin. In a process called ‘treadmilling’…. The
filament will ‘move’ to the right

Question 28

Myosins

Answer 28

are the motor proteins of microfilaments

are a large family of
motor proteins that move along actin
filaments, while hydrolyzing ATP.

All myosins share a characteristic
motor head for binding actin and
hydrolyzing ATP.

The myosin tail is divergent.

Myosins can be divided into two
groups:
Conventional (type II) myosin
Unconventional myosins (I, V, VI, VII, XV)
All move to the + end of
microfillaments, except myosin VI
(there’s always an exception!)

Question 29

Myosin II

Answer 29

found in muscle cells- most common type 
includes two heavy chains,
each with a globular motor domain that
includes a binding site for ATP and a
domain that interacts with actin

Question 30

Myosin V

Answer 30

V has two heavy chains like
myosin II, but myosin V has a longer
neck region and a cargo binding domain