Microtubules

Question 1

Major roles.

Answer 1

1) transport pathway in cells (interphase cells, mitotic dividing cells, cilated cell, nerve cell)

Question 2

General

Answer 2

Most of the time, its skeleton exists in in the interphase arrangement where the tubules are nucleated close to the nucleus and run out like trackways usually as individual microtubules to the periphery of the cell.

During MITOSIS, the tubules are DISASSEMBLED and the components and associated proteins are utilised to generate the mitotic spindle

Question 3

localisation

Answer 3

Typically organised from the microtubule organising center or centrosome and they run from the perinuclear region to the periphery.

Question 4

Cell cilitated/ flagella (specialised cells)

Answer 4

Microtubules important in generating beating motion. Allows bacterium to swim in water. In the tracea, there are multiple projections

Question 5

Neuronal cells

Answer 5

They are released from the centrosome and move into axon

Question 6

structure 1

Answer 6

Microtubules in all cell type all SHARE TUBULUAR structure with multiple subunits, usually 13 in an individual microtubule

exist in Mammalian cells as individuals; neuronal axons

all the activity happens on the activity happens on the OUTSIDE of the tubes

Question 7

model of transport

Answer 7

cells from fish and reptile aggregate PIGMENT; Pigment in dispersed state in cell (cell dark) or aggregate to center (lighter); pigment runs along microtubules.

Cells stimulated with cAMP increases dispersion; no cAMP causes aggregation

Model shows intracellular vesicles moving along tubule skeletons.

Question 8

Secretory pathway

Answer 8

Where proteins are made in the E.R; post translational in the Golgi apparatus (path 1) and then to cell periphery (path 2)

path 1 and 2 depends on microtubules (also determines the position of the organelles)

Question 9

Why is it important ?

Answer 9

Without tubules, structure of whole cell and it activity depletes completely.

Allows cells to polarise themselves so they can act in a particular way the the reorganisation of the cytoplasm

Question 10

Immune system

Answer 10

antigen presents itself; interacts with the T cell; this interaction is driven my the correct ORIENTATION of the microtubule (cell polarises)

Question 11

Key Processes

Answer 11

Swimming,

mitosis

Question 12

Structure 2

Answer 12

long Tubes-25 nm across
contain 13 proto-filaments of alternating alpha and beta tubulin forming a heterdimer dimer (SUBUNIT= dimer made of made up of one alpha and one beta tubulin- already in this form in the cytoplasm) subunits added to elongate the filament.

Doublet where one microtubule growing on the side of another

triplet structures (in basal bodies)

Question 13

Proteins that form it and associated with it. What role do the associated proteins have on tubule?

Answer 13

Tubulin
motor proteins (transport material)
Taq (for regulating polymersation) which has been implicated in Alzheimers disease.

associated proteins determine tubule spacing in different parts of the systems.

Question 14

GTP mechanism

Answer 14

each dimer binds to GTP; There are two types of GTP present in the dimer

1) GTP where it is accessible as it is free so it can be hydrolysed
2) GTP in a non hydrolysable form trapped in hetrodimer

Question 15

What is the result of having an altering alpha and beta structure?

Answer 15

causes a difference in polarity; one end always has a beta exposed, one end that always has an alpha exposed

Question 16

GTP 3

Answer 16

When subunits ASSOCIATE they have to be in a GTP-bound form, after association subunit-GTP hydrolysed to GDP

Question 17

Describe intermediate forms seen in vitro

Answer 17

1) activation stage: shown by the generation of rings from individual subunits.
2) elgongation/rapid growth
3) steady state phase. This depends on the CONCENTRATION of TUBULIN; net addition and removal at each ed (treadmilling here)

Nucleation step controlled by the MOC (organising center)

Question 18

What allows tubule to be flexible, reorientate cytoskeleton?

Answer 18

highly dynamic structures: Tubules can either be shrinking (sometimes they shrink right back or growing (constantly) or resting. PHOTOBLEACHING

PROCESS OF RESCUING- where it needs to grow.

Question 19

What drives the dynamic nature of tubulin?

Answer 19

The exchangable GTP/GDP in the beta tubulin

Question 20

How can microtubules be orientated

Answer 20

tissue in buffer of free tubulin which ass.with tubulin FORMING HOOKS

Electron microscope shows hooks are either orientated clockwise or anticlockwise. The direction of hooks in the anticlockwise direction shows which tubulin end is positive.

Question 21

What cell type has most microtubules?

Answer 21

pillar cell in the ear.

Question 22

Microtubule organising center (mammalian centromosome)

Answer 22

2 components: centrioles (triplet tubules) and pericentriolar material (PCM) (diffuse material that nucleates the tubule)

Question 23

centriole structure

Answer 23

9-fold symmetry; 9 triplet microtubules

Question 24

Pericentriole material

Answer 24

contains structures called gamma tubulin ring complexes (a type of tubulin) which acts as the nucleation site IN MOST CELLS as well as severing and anchoring activities.

Question 25

Which cell type does not have MOC

Answer 25

nerve cells; they are nucleated in the cell body and released, moving along into dentrites.

Fat cells also have free microtubules.

Question 26

What is photobleaching.

Answer 26

Fluorscent labels used

Slowly meshwork recovers, through new tubulins being disaasemble and reassembled; some diappear faster than other as they behave differently

Question 27

Explain the relevance of affinity and binding.

Answer 27

polymerisation at plus end by addition of subunits.

Two affinities at the different ends of the tubule; the minus end binds the MOC and so less growth at this end.

Question 28

GTP mechanism 2

Answer 28

Hydrolysis of GTP after addition: determines structure.

Fast growing tubule has a GTP cap. (slow growing has GDP were subunits disassoicate and subunits break away)

The change from GTP to GDP alters the conformation of the tubulin

Question 29

Describe regulation by mRNA stabilising mechanism

Answer 29

regulated by an interaction with mRNA and the translation.

As the polypeptide of tubulin is made, the 4 terminal aa interact with free tubulin.

This causes the translation of the message to be stalled, mRNA is cleaved and, NO MORE tubulin made.

Ass. proteins also regulate the amount of tubules at the MOC (determined by the number of rings; nucleation sites.

At positive end, crosslinking, capping proteins regulate turn over of tubule at the end.

actin and microtubulin regulate eachother too; tubules signal to the actin and activate their activity

Question 30

motor families

Answer 30

1)kinesin - Move from middle to membrane (E.R and mt) PLUS END DIRECTED MOTOR

There is a kinesin that allows motor to bind to two tubules; important in mitosis.

2)dynein - move from membrane to middle. MINUS END MOTOR. It cargo domain allows it to interact with other components (DYNEIN COMPLEX) It is a MINUS END motor

ATP activated; motors change conformation by hydrolysing ATP to ADP So can change its position.

two heads: one on the tubule and one on the change its conformation via hydrolysis and “step over”

pigment move both directions

structurally share have a motor domain (on the tubule) and cargo domain as well as a linker region.

Question 31

types of dynein motors

Answer 31

flagella dynein

cytoplasmic dynein

Question 32

Describe mechanism of the motors

Answer 32

1) bind to vesicle in the cargo domain
2) then walks along the tubule (nothing in tubule center)
3) orientation of microtubule recognised by motors by the subunits

Question 33

How does the microtubules influence the EXTRAcellular movement?

Answer 33

Using cillia, or cilliated epithelia (broncus, trachea, sperm mobility)

In cilium there is a 9 +2 motor ; 2 dyein motors to 9 adjacent tubules (so they can slide over eachother. This generates it movement.

Question 34

Why is Mitosis segregation via microtubules important?

Answer 34

If a part of the chromasome is lost has severe genetic defects.