Exam 4 Content - MCB 252

What is a microtubule?

A hollow cylindrical cytoskeletal filament composed of tubulin

What proteins make up microtubules?

Alpha-tubulin and beta-tubulin

What is the basic building block of a microtubule?

An alpha-beta tubulin heterodimer

Are tubulin dimers stable?

Yes they rarely dissociate

How many protofilaments form one microtubule?

13

What shape is a microtubule?

A hollow tube

Which end of a microtubule grows faster?

The plus end

Which end is usually anchored at the MTOC?

The minus end

Which nucleotide binds free tubulin?

GTP

Which tubulin subunit hydrolyzes GTP?

Beta-tubulin

What happens after beta-tubulin hydrolyzes GTP?

GTP becomes GDP making the microtubule less stable

What is the GTP cap?

A region of GTP-bound tubulin at the growing plus end

Why is the GTP cap important?

It stabilizes the microtubule

What causes catastrophe?

Loss of the GTP cap

What is catastrophe?

Rapid microtubule depolymerization

What is rescue?

The return from shrinkage to growth

What is dynamic instability?

Repeated switching between growth and shrinkage

Why is dynamic instability important?

It allows rapid remodeling of the microtubule cytoskeleton

What is treadmilling?

Addition of tubulin at the plus end and loss at the minus end

What determines microtubule polarity?

The head-to-tail arrangement of alpha-beta tubulin dimers

What is found at the center of an interphase cell?

The centrosome

What is the centrosome?

The major microtubule organizing center of interphase cells

What does centrosome mean?

Body in the center of the cell

What happens to the interphase microtubule cytoskeleton during mitosis?

It is completely disassembled and rebuilt

What replaces centrosomes during mitosis?

Spindle poles

What is the function of spindle poles?

Organize the mitotic spindle

What is the microtubule organizing center in cilia?

The basal body

What is the microtubule organizing center in flagella?

The basal body

What structures use basal bodies?

Cilia and flagella

Name five structures built from microtubules.

Interphase cytoskeleton mitotic spindle cilia flagella and Paramecium

Which microtubule ends point toward the edge of the cell?

The plus ends

Which microtubule ends remain anchored?

The minus ends

What are the three classes of spindle microtubules?

Kinetochore polar and astral microtubules

What are kinetochore microtubules?

Microtubules connecting spindle poles to kinetochores

What is a kinetochore?

The chromosome region responsible for segregation

Where are the minus ends of kinetochore microtubules?

At the spindle poles

Where are the plus ends of kinetochore microtubules?

Attached to kinetochores

What are polar microtubules?

Microtubules that overlap with those from the opposite spindle pole

Do polar microtubules attach chromosomes?

No

Where do polar microtubules overlap?

Near the center of the spindle

What are astral microtubules?

Microtubules radiating from spindle poles toward the cell cortex

Which spindle microtubules extend toward the cell cortex?

Astral microtubules

Which spindle microtubules attach chromosomes?

Kinetochore microtubules

Which spindle microtubules generate pushing forces?

Polar microtubules

Do all three spindle microtubule classes work together?

Yes

What is the overall function of the mitotic spindle?

To segregate chromosomes accurately

How do cilia differ from microvilli?

Cilia are motile whereas microvilli are not

What powers movement of cilia?

Microtubules

What powers movement of flagella?

Microtubules

Where are the minus ends in cilia?

Anchored in the basal body

Where are the plus ends in cilia?

Toward the tip

What is the function of the basal body?

Anchor and organize ciliary microtubules

What technique first revealed microtubules?

Electron microscopy

When were microtubules first defined?

1963

What does tubulin refer to?

The alpha-beta heterodimer

What property allows microtubules to rapidly reorganize?

Dynamic instability

Which end is generally more dynamic?

The plus end

What is polymerization?

Addition of tubulin dimers to a microtubule

What is depolymerization?

Loss of tubulin dimers from a microtubule

What is required for microtubule assembly?

GTP-bound tubulin dimers

Why is GDP-tubulin less stable?

Because GTP hydrolysis changes tubulin conformation

What ultimately causes protofilaments to peel apart?

Loss of the stabilizing GTP cap

What is the relationship between GTP hydrolysis and catastrophe?

Hydrolysis precedes loss of the cap and catastrophe

What cellular structures depend on microtubules?

The interphase cytoskeleton mitotic spindle cilia and flagella

What is the major function of the interphase microtubule cytoskeleton?

Organization and intracellular transport

Why are spindle poles considered equivalent to centrosomes?

Both function as microtubule organizing centers

What changes between interphase and mitosis?

The microtubule array is reorganized into the mitotic spindle

Why are microtubules considered polarized?

Their two ends have different structures and behaviors

Which end is associated with growth toward the cell edge?

The plus end

Which end is associated with the centrosome?

The minus end

What are microtubule associated proteins called?

MAPs

What are the major functions of MAPs?

Assembly disassembly stabilization capping spacing and arrangement of microtubules

What protein binds specifically to the GTP cap?

EB-1

What does EB stand for?

End Binding

Where does EB-1 bind?

The GTP cap at the growing plus end

What is the primary function of EB-1?

Stabilize the GTP cap

Does EB-1 remain stationary?

No it travels with the growing plus end

Besides stabilizing the cap what else does EB-1 do?

Recruits other proteins to the growing microtubule

Which proteins promote microtubule disassembly?

Stathmin and kinesin-13

How does stathmin promote depolymerization?

By promoting the curved protofilament conformation

How does kinesin-13 promote depolymerization?

By using ATP hydrolysis to bend protofilaments

Does kinesin-13 function as a transport motor?

No it depolymerizes microtubules

Which protein severs microtubules?

Katanin

How does katanin differ from kinesin-13?

Katanin cuts microtubules internally instead of removing subunits from the ends

What are the three major functions of the centrosome?

Nucleation capping and anchoring of microtubules

Who first visualized centrosomes?

Flemming and Van Beneden

When were centrosomes first observed?

1875

What is the pericentriolar material?

Protein-rich material surrounding centrioles

What abbreviation is used for pericentriolar material?

PCM

Do both centrosomes and spindle poles contain PCM?

Yes

Which tubulin family member is located in the PCM?

Gamma-tubulin

How was gamma-tubulin discovered?

Genetic studies in yeast

Where is gamma-tubulin localized?

The pericentriolar material of centrosomes

What happens if antibodies against gamma-tubulin are added?

Microtubule assembly is blocked

What complex contains gamma-tubulin?

Gamma-tubulin ring complex

What abbreviation is used for the gamma-tubulin ring complex?

Gamma-TuRC

What are the functions of Gamma-TuRC?

Microtubule nucleation minus-end capping and anchoring

Where are Gamma-TuRCs located?

Within the PCM

Do centrioles directly nucleate cytoplasmic microtubules?

No Gamma-TuRCs do

What role do centrioles play?

They organize the centrosome