Cytoskeleton

Question 1

What roles does the cytoskeleton play in the cell?

Answer 1

Maintains cell structure

Involved in nearly every cell process

Question 2

What are the three main types of cytoskeleton?

Answer 2

Microtubules (hollow interiors, made of single type of proteins)
Actin Filaments (made of single type of proteins)
Intermediate Filaments (made of many types of proteins)
There is a fourth type, but little is known about it now.

Question 3

What are microtubules constructed from?

Answer 3

Microtubules are constructed from heterodimers of alpha and beta tubulin proteins. There are at least 8 isoforms of beta tubulin that are expressed in different ratios in different tissues. These isoforms differ in their tail domain structure, possibly inferring different capabilities on the microtubule.

Question 4

What differentiates the functions of GTP and ATP?

Answer 4

ATP is used primarily to provide energy to a system. GTP is converted to GDP as a signaling process. It does not typically provide energy, only an on/off switch-like signal that changes the conformation of associated proteins and alters their binding.

Question 5

How and why are microtubule filaments stabilized?

Answer 5

Microtubules are very dynamic structures that are short lived. Constructed of alpha/beta heterodimers, with the beta unit on “top”, the GTP bound to the alpha does not readily hydrolyze, but the beta GTP does. When the beta GTP -> GDP, the heterodimer kinks, interrupting the tubule structure and allowing it to break apart. By placing a number of heterodimers with GTP at the “top” of the microtubule, a “GTP cap” is formed which prevents the disintegration of the microtubule.

Question 6

How can microtubules be degraded and what disorder is associated with their degradation?

Answer 6

Removal of the microtubule cap-binding proteins allows the beta-GTP cap to hydrolyze to GDP and the microtubule disintegrates. Alternately, and faster, there are microtubule severing proteins (katanin, spastic, fidgetin) that cut the microtubule below the GTP cap and degrade it that way. Hereditary spastic hyperplasia is associated with severing protein disorders.

Question 7

What are the known disease caused by mutations in the microtubule severing proteins?

Answer 7

There are no known mutations in the genes for fidgetin or katanin, disorders in katanin are not viable because it acts in mitosis. Mutations in spastin result in hereditary spastic hyperplasia

Question 8

How do mutations in spastin result in hereditary spastic hyperplasia?

Answer 8

The axons in nerve cells contain microtubules that run their entire lengths. These are carefully arranged to in normal cells and allow the transmission of proteins and neurotransmitters from the cell bodies to the synapses. Disorders in synaptin cause mis-alignments of these micro tubules resulting in ineffective transmission of neuron signals.

Question 9

How are SNARE proteins and microtubules similar?

Answer 9

They are both deconstructed via triple ATPases formed into hexamers.

Question 10

What are the four primary functions of microtubules?

Answer 10

Cellular cytoskeleton
Intracellular transport
Cell division
Cilia

Question 11

Why do disorders in microtubules or microtubule severing proteins first show in neurons?

Answer 11

All cells depend upon the the structure of microtubules emanating from a centrosome and proceeding out to target locations. Due to their long axons in need of precisely targeted microtubules, neurons are much more sensitive to disruptions in the microtubule system, thus they manifest diseases more quickly. Anything that affects transport along microtubules (the microtubules, kinesins, or adaptor molecules) can manifest neuropathies.

Question 12

What compounds move along microtubules

Answer 12

Kinesins (move towards + end)

Dyneins (move towards - end) Large complex, 50+ proteins Only one copy of dynein in cells, no viable mutations.

Question 13

How is vesicle transport along microtubules regulated?

Answer 13

There are 18 different kinesin molecules, but over 100 vesicle targeting proteins. However, there are adapter molecules that bind to specific vesicles and specific kinesins, providing the necessary specificity of transport. Regulation of these adapter molecules provides regulation of transport.

Question 14

What is the structure of kinesin molecules?

Answer 14

Most kinesin molecules are homodimers containing long coiled coil sections, the head domain binds to the microtubule, and the tail domain binds to cargo via adaptor proteins. Kinesins move vesicles, RNA, proteins, and other microtubules.

Question 15

What is uniques about the structure of kinesin molecules that move microtubules and what is one example of their location?

Answer 15

They contain two head domains, because the head domains bind to the microtubules. They are active during cell division.

Question 16

How is ATP used by kinesins to move along the microtubules?

Answer 16

ATP is not used as a energy source, but only to affect conformation of the head domains. When ATP is bound to the kinesin head domain, it binds to the microtubule, when ATP is hydrolyzed to ADP the head domain does not bind to the microtubule. The ratio of ATP:ADP is always 50:50, less than that would result in complete dissociation of the kinesin, more than that would result in immobility of the kinesin.

Question 17

Why is retrograde transport important for development and continued sustenance of the neuron cell?

Answer 17

During development, neurons extend axons out in search of targets to innervate. When a target is found, growth factors are secreted and taken up by the axon to signal the neuron to survive. If the neuron does not find a target, or if the retrograde transport is dysfunctional, the growth factors are not received and the neuron dies.

Question 18

What is the mitotic spindle composed of?

Answer 18

The mitotic spindle consists of a centrosome organizing the microtubules. Cell division is the only case where two centrosomes may exist in a cell.

Question 19

Three types of microtubules during cell division.

Answer 19

Astral microtubules (Connect to the cell membrane)
Kinetochore microtubules (Connect to the chromosomes)
Central spindle microtubules (overlap each other)

Question 20

What role to central spindle microtubules play and how can they be targeted in drug therapy?

Answer 20

These microtubules actually drive the dividing cells apart, thus mutations in the kinesins that act here are completely lethal. Also, they are targets for chemotherapy drugs because destabilizing or over-stabilizing them both lead to arresting cell division.

Question 21

Four types of intermediate filaments and their locations

Answer 21

Neurofilaments - neurons
Keratins - epithelia (forms heterodimers of acidic/type-1 and basic/type-2 keratin)
Vimentin - connective tissue, muscle, neuroglial cells
Nuclear lamins - all animal cells

Question 22

What is the basic structure of intermediate filaments, and where do most mutations occur?

Answer 22

They are non-polarized (unlike tubulins), so no + or - ends. A-helix chains form parallel (COOH+COOH) coiled coil dimers, dimers form antiparallel tetramers. Many mutations occur in the head domains, allowing the formation of the tetramers but not larger structures made from tetramers.

Question 23

Why are hepatocytes more sensitive to intermediate filament mutations?

Answer 23

They only express one type of keratin whereas other cells produce several. If the hepatocyte cells are contain a mutation in their keratin (K8 or K18), they cannot substitute another keratin filament.

Question 24

Diseases associated with nuclear lamina disfunction

Answer 24

Lipodystrophy - inability to form adipose cells
Mandibuloacral dysplasia - disease affecting joints
Progeria syndrome
Large variety of myopathies

Question 25

Common mutation in nuclear lamina leading to diseases

Answer 25

Nuclear lamina filaments have prenylated CAX Box regions that insert themselves into the plasma membranes during translation in the ER. Normally the nuclear lamina proteins are clipped by special proteases, allowing the proteins to become cytosolic. Mutations in these membrane bound regions prohibit the cleaving of the proteins, keeping them attached to the membrane.