Cytoskeleton I

Question 1

1. Discuss the concept of a cytoskeleton.
2. Describe microtubule and intermediate filament cytoskeleton (their properties, their functional roles, and their protein composition).
3. Discuss cytoskeletal dynamics and the role of certain proteins and drugs in tubulin polymerization/depolymerization.
4. Explain the concept of molecular motor. Explain the mechanisms of tubulin-based movement and intracellular transport.
5. Discuss the role of microtubules in mitosis.
6. Discuss the cytoskeleton in the context of disease processes.

Answer 1

x

Question 2

Functions of the cytoskeleton (6)

Answer 2

1) cell shape
2) mechanical strength
3) structures needed for locomotion
4) support for the plasma membrane
5) scaffold for the spatial organization of organelles
6) means for intracellular transport of organelles and other cargo

Question 3

3 families of structures:

Answer 3

microfilaments, microtubules, and intermediate filaments.

[Microfilaments = actin cytoskeleton]

Question 4

Microtubules (MTs) function primarily as:

Answer 4

1) scaffolds for the spatial organization of organelles
2) organelle movement
3) movement of cilia and flagella.

Question 5

Microtubules are typically attached at one end to the _____, also known as ______, which is made up of _______ and is typically located near the _____.

Answer 5

Centrosome; perinuclear microtubule organizing center (MTOC); 2 centrioles; nucleus

Question 6

Microtubules are made of 13 parallel ______, which themselves are composed of heterodimeric repeats composed of _____.

Answer 6

protofilaments; tubulin alpha and tubulin beta;

Question 7

These protofilaments have a + and - end based on ____?

Answer 7

Whether the alpha or beta subunit is exposed.

Question 8

GTP-bearing β subunits favor _____, so that end of the tubule, known as ______, is the one that _____.

The opposite end, or ____, tends to be _____.

Answer 8

polymerization; plus-end; grows

minus-end; disassembling or shrinking

Question 9

What regulates stability at the + end?

Answer 9

The accessibility of the GTP. When the B subunit is bound to GTP, it is stable. Hence the idea of a “GTP cap.” When bound to GDP, however, the B subunit “kinks” which causes rapid depolymerization from the + end.

Question 10

What differentiates the 8 isoforms of the B subunit? What is the purpose of these structures?

Answer 10

The tails. The tail domain seems to help the cleavage proteins initiate cleavage. Without the tail domains, MTs cannot be readily disassembled.

Question 11

_____ form the basal bodies of cilia and flagella.

Answer 11

Centrioles

Question 12

The GTP attached to the alpha subunit is (exposed/not exposed)

Answer 12

not exposed

Question 13

Two microtubule severing proteins were mentioned in class. What were they? For which were we given a clinical correlate?

Answer 13

katanin, spastin. A mutation in spastin causes Hereditary spastic paraplesia.

[without spastin the parallel microtubule tracks in neurons become disorganized and neurotransmitter transport to the synaptic junction is compromised]

Question 14

Two different classes of MT motors exist, ____, which move cargo toward the plus-end, and ______, which move cargo toward the minus-end.

Answer 14

kinesins; dyneins

[Kinesins require adapters to bind cargo. Not sure if dyneins do]

Question 15

When the head of the motor is bound to ATP, it _____ MT. When it is bound to ADP, it _______ MT.

Answer 15

binds; does not bind

Question 16

Molecular motors made of heterodimers _______ feet to move along the MT. Homodimers _____ feet to move.

Answer 16

step same foot forward each time; swing alternate feet in front of each other

Question 17

Three types of MTs can be distinguished:

In all cases, MT plus-ends point _____ from the centrosomes (or spindle poles).

Answer 17

1) astral MTs that radiate out from the centrosomes;
2) kinetochore MTs that are attached to the kinetochore formed at the centromere of each duplicated chromosome
3) overlap MTs that interdigitate at the equator of the spindle.

away

Question 18

Please describe the structure of intermediate filaments, beginning with the smallest subunit and working upwards.

Answer 18

Alpha helical coils bind in parallel dimers. These then form coiled coils in antiparallel fashion to form a dimer (of coiled coils). Two dimers align at opposite poles to form a tetramer of coiled coils (8 individual subunits).

Eight tetramers align side by side (16 individual subunits) that overlap with another eight tetramers such that each section has a total of 32 helices that provide great tensile strength.

Question 19

_____ are present in a majority of cell types.

Answer 19

Vimentins

Question 20

______, a family of about 50 proteins, are dominant components of the epidermis and its appendages, providing mechanical strength.

Answer 20

Keratins

Question 21

_____ (three types) co-assemble to form_____, which are found in high concentration in vertebrate axons.

Answer 21

Neurofilament proteins; neurofilaments

[Neurofilament abundance appears to control axonal diameter.]

Question 22

______ is the IF protein characteristic of astrocytes in the CNS.

Answer 22

Glial fibrillary acidic protein (GFAP)

[Therefore, GFAP IFs are abundant in connection with inflammatory and/or degenerative processes in the brain. For example, Alzheimer plaques are surrounded by GFAP-rich reactive astrocytes.]

Question 23

Keratin mutations may interfere with filament assembly. The resulting epidermis is highly sensitive to mechanical stress and blisters easily, causing a severe disorder called _____-. Mutations in some of the associated proteins (e.g., those that anchor the filaments in desmosomes) may result in similar clinical syndromes.

Answer 23

epidermolysis bullosa simplex

Question 24

Neurofilaments: Mutations in the light chain may interfere with axonal transport of neurofilament subunits and cause a peripheral neuropathy called ______.

Answer 24

Charcot-Marie-Tooth syndrome

Question 25

Abnormal neurofilament assembly seems to be involved in the neurodegenerative disease_______.

Answer 25

amyotrophic lateral sclerosis (ALS or Lou Gehrig’s Disease)

Question 26

Mutations in lamins can result in nuclear instability. Lamin mutations are linked to _____. This is caused by a dominant negative product caused by a mutation that prevents cleavage near the c terminus of a protein. CAAX remains bound to the membrane.

Answer 26

progeria

Question 27

microtubules:
structure
predominant protein
function

Answer 27

structure: tubular, 25 nm diameter
predominant protein: alpha and beta tubulin
function: movement of flagella and cilia, scaffold for cell organization and movement of organelles (including chromosomes)

Question 28

intermediate filaments:
structure
predominant protein
function

Answer 28

structure: complex rope, 10 nm diameter
protein composition: vimentin, keratin, neurofilament protein, others
fxn: mechanical stability

Question 29

microfilaments:
structure
predominant protein
function

Answer 29

helical filament, 5-9 nm diameter
actin
cell movement, contraction

Question 30

Most cell types have multiple kinesins, so loss or mutation of one does not lead to significant pathology. For _____ however, only K8/K18 are expressed making people with mutations in these genes prone to _____.

Answer 30

hepatocytes; liver failure

Question 31

What are kinesins often used as, clinically?

Answer 31

Markers for cancer. Cells maintain original expression of kinesins after metastazising, so it may be possible to tell where a cancer originated based on expression of these markers. (Or so I gathered from lecture :)