Class 19 - Microtubules and Motor Proteins (PRETTY GOOD)

Question 1

Tubulin

Answer 1

Subunit of microtubules. Made of 2 globular proteins (a and b tubulin). Each subunit has 2 binding sites for GTP.

One on b tubulin can be GTP or GDP, but a’s remains constant.

13 filaments. Longitudinal contacts (b and a), and lateral (a-a, b-b).

Question 2

Dynamic instability

Answer 2

Rapid interconversion between growing and shrinking state, at a uniform free tubulin concentration.

Question 3

Catastrophe

Answer 3

Change from growth to shrinkage from random loss of GTP cap.

Question 4

Rescue

Answer 4

Change from shrinkage to growth from random regaining of GTP cap.

Question 5

Microtubule-organizing center (MTOC)

Answer 5

Specific intracellular location where microtubules are generally nucleated. Protect minus end.

Highest concentration of y-tubulin (helps assembly).

Question 6

y-tubulin ring complex (y-TuRC)

Answer 6

Nucleates assembly and remains associated with minus end. Had proteins that bind directly and directly to y-tubulin.

Question 7

Centrosome

Answer 7

A microtubule organizing center (MTOC) located adjacent to the nucleus from which microtubules nucleated from minus end.

Not absolutely required for nucleation, can also occur in cytoplasm.

Question 8

Centrioles

Answer 8

Cylindrical structured embedded in centrosome. Modified microtubules where nucleation takes place.

Most plant and fungi dont have these but still use y-TuRC.

Question 9

Microtubule-associated proteins (MAPs)

Answer 9

Bind to microtubules. Mediate interactions with cells, stabilize against disassembly.

MAP2 - long projecting domains
Tau - Short projecting domains
Augmin - Nucleates microtubule branching, recruits y-tubulin.

Question 10

Kinesin

Answer 10

Motor proteins which regulate MICROTUBULE dynamics and move toward plus end.

Also - Mitotic spindle formation and chromosome separation

Select appropriate cargo using tail and receptors on organelle.

Question 11

Dynein

Answer 11

MICROTUBULES. Largest known motors. Move toward minus end.

Couple ATP hydrolysis and force-generating conformational change to microtubule binding.

2 types:
Cytoplasmic - Moves organelles, centrosome/nucleus positioning, mitotic and meiotic spindle

Axonomic - Cilia and flagella

Select appropriate cargo with dynactin and receptors on organelles.

Question 12

Flagella

Answer 12

Enable cells to swim through liquid media using undulating motion.

Question 13

Cilia

Answer 13

Enable movement of cell or of fluid over surface by beating in whiplike fashion.

Question 14

Axoneme

Answer 14

The core of a cilia or flagella, responsible for the movement by its bending.

Composed of microtubules and associated proteins. Arranged in ring around a pair of microtubules.

Dynein forces adjacent doublets to slide relative to one another, and force converted into bending motion.

Question 15

Primary cilia

Answer 15

Shorter, nonmotile counterpart of cilia and flagella. Specialized compartments of organelles which perform a range of functions. Found on surface of most cells; respond to exterior environment.

Question 16

Microtubules + 4 functions

Answer 16

Structure: a-tubulin + b-tubulin (heterodimer), 13 protofilaments

A-tubulin at minus end and b at plus end. Plus end grows and shrinks more rapidly. GDP bound have low affinity for one another, higher cc level than actin—>disassemble more easily

1. Positions membrane bound organelles
2. Direct intracellular transport
3. Support
4. Mitotic spindle.

Question 17

Catastrophe factors

Answer 17

Bind to microtubule ends and pry protofilaments apart, lowering normal activation energy barrier that prevents shrinking

Question 18

XMAP215

Answer 18

Stabilizes plus end, binding subunits and accelerating polymerization.

Question 19

Plus-end tracking proteins (+TIPs)

Answer 19

MAPs which bind to an actively growing plus end and dissociate when it begins to shrink. Link microtubules to other structures.

Question 20

Stathmin

Answer 20

MICROTUBULES. Sequesters subunits, making them unavailable for polymerization, causing catastrophe.

Question 21

Katanin

Answer 21

Severs MICROTUBULES. Also releases them from MTOC.

Sometimes helps promote growth by letting GTP in where it removed GDP-bound subunits from the side.

Question 22

Motor protein 3 functions

Answer 22

1. Move cargo such as organelles and macromolecules over long distances.
2. Slide microtubules relative to one another (cilia beating, muscle contraction)
3. Regulate microtubule dynamics

Question 23

Kinesin-13

Answer 23

MICROTUBULES. Induces catastrophe and disassembly (catastrophe factor).

Question 24

What places in the cell are microtubules found? (4)

Answer 24

1. In interphase cell: Star-like cytoplasmic array
2. Mitotic cell: mitotic spindle
3. Cilia and flagella
4. Axons

Question 25

2 domains of motor proteins

Answer 25

1. Motor domain (“feet”) - binds and hydrolyzes ATP, interacts w filaments, determines movement direction
2. Tail domain (“head”) - binds to cargo, determines cargo identity

Question 26

Motor protein cycle (5 steps)

Answer 26

1. Bind to filament (release of ADP, ATP enters)
2. Conformational change
3. Filament release (throws foot forward).
4. Conformational change (release of phosphate—>ADP)
5. Filament rebinding (release ADP, ATP enters).