Microtubules Flashcards
Major roles.
1) transport pathway in cells (interphase cells, mitotic dividing cells, cilated cell, nerve cell)
General
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
localisation
Typically organised from the microtubule organising center or centrosome and they run from the perinuclear region to the periphery.
Cell cilitated/ flagella (specialised cells)
Microtubules important in generating beating motion. Allows bacterium to swim in water. In the tracea, there are multiple projections
Neuronal cells
They are released from the centrosome and move into axon
structure 1
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
model of transport
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.
Secretory pathway
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)
Why is it important ?
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
Immune system
antigen presents itself; interacts with the T cell; this interaction is driven my the correct ORIENTATION of the microtubule (cell polarises)
Key Processes
Swimming,
mitosis
Structure 2
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)
Proteins that form it and associated with it. What role do the associated proteins have on tubule?
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.
GTP mechanism
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
What is the result of having an altering alpha and beta structure?
causes a difference in polarity; one end always has a beta exposed, one end that always has an alpha exposed
GTP 3
When subunits ASSOCIATE they have to be in a GTP-bound form, after association subunit-GTP hydrolysed to GDP
Describe intermediate forms seen in vitro
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)
What allows tubule to be flexible, reorientate cytoskeleton?
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.
What drives the dynamic nature of tubulin?
The exchangable GTP/GDP in the beta tubulin
How can microtubules be orientated
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.
What cell type has most microtubules?
pillar cell in the ear.
Microtubule organising center (mammalian centromosome)
2 components: centrioles (triplet tubules) and pericentriolar material (PCM) (diffuse material that nucleates the tubule)
centriole structure
9-fold symmetry; 9 triplet microtubules
Pericentriole material
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.
Which cell type does not have MOC
nerve cells; they are nucleated in the cell body and released, moving along into dentrites.
Fat cells also have free microtubules.
What is photobleaching.
Fluorscent labels used
Slowly meshwork recovers, through new tubulins being disaasemble and reassembled; some diappear faster than other as they behave differently
Explain the relevance of affinity and binding.
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.
GTP mechanism 2
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
Describe regulation by mRNA stabilising mechanism
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
motor families
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.
types of dynein motors
flagella dynein
cytoplasmic dynein
Describe mechanism of the motors
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
How does the microtubules influence the EXTRAcellular movement?
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.
Why is Mitosis segregation via microtubules important?
If a part of the chromasome is lost has severe genetic defects.
