Week 10 - The Cytoskeleton and Microtubules Flashcards
What are the 5 functions of the cytoskeleton?
- Provides structural support
- Positions organelles
- Directs vesicular transport
- Involved in locomotion
- Required for cell division
What are the 4 main structures of the cytoskeleton?
- Actin filaments
- Intermediate filaments
- Microtubules
- Motor proteins
What are the 3 types of filaments that form the cytoskeleton?
- Microfilaments (actin filaments)
- Intermediate filaments (intermediate filament proteins)
- Microtubules (Tubulin)
What is immunofluorescence and what is it used for?
It is a technique used to determine the location of proteins within the cell, not just the cytoskeleton.
How is immunofluorescence done?
- Antibody is used which binds to the protein of interest
- A second antibody binds to the first and is covalently tagged with a fluorescent molecule
- A fluorescence microscope is used to excite the fluorescent molecule and visualize the light emitted.
Why does the light microscope have a resolution limit?
Due to diffraction; it is based on the wavelength of light.
Why does the electron microscope have a higher resolution than the light microscope?
It uses electrons, which have a shorter wavelength than light; can therefore see clearer.
For cell motility and crawling, what must the actin filaments rapidly do?
They must rapidly disassemble and reassemble at the leading edge.
Most interphase microtubules radiate from what?
They radiate from one microtubule organizing centre.
What are microtubules reorganized to form in dividing cells?
Bipolar mitotic spindles.
A microtubule is made up of what?
13 protofilaments of tubulin in a hollow cylinder formation.
Microtubules are involved in what 5 things?
- Intracellular transport
- Structural support
- Cell organization
- Mitosis (mitotic spindle)
- Cell motility (flagella and cilia)
Microtubules are inextensible. Does this mean they don’t extend?
No, microtubules do grow and shrink. Inextensible means that they are not elastic.
Microtubules are made up of individual subunits of what 2 things?
- Alpha-tubulin
2. Beta-tubulin
Together, Alpha-tubulin, Beta-tubulin, and GTP form what?
A tubulin heterodimer.
Tubulin heterodimers bind together to form what?
Form a protofilament.
The regular arrangement of alpha and beta subunits give the microtubule what?
Polarity; Beta is a plus end and alpha is a minus end.
What is at the minus end of a microtubule?
An alpha-tubulin subunit.
What is at the plus end of a microtubule?
A beta-tubulin subunit.
What happens to GTP is a heterodimer has been attached to a microtubule for a while?
GTP is cut to GDP; usually by the Beta-tubulin.
How do GTP caps form on microtubules?
GTP caps form as new heterodimers (with uncut GTP) add onto the plus end (beta-tubulin).
How is the plus end of a microtubule stabilized as to not lose heterodimers?
The minus end is stabilized by microtubule organizing centres (MTOCs).
Where does all loss and addition occur on a microtubule?
At the plus end.
What is an MTOC and what is it’s function?
Microtubule organizing centre; structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions:
- the organization of eukaryotic flagella and cilia
- the organization of the mitotic and meiotic spindle apparatus.
What does an MTOC consist of?
Made up of a pair of centrioles at its centre, and is surrounded by pericentriolar material (PCM).
What kind of bonds are between individual subunits of a protofilament?
All bonds are non-covalent.
Where are the non-covalent bonds the strongest in a microtubule?
Within the heterodimers.
A heterodimer just added to the end of a protofilament while be what form?
It will be t-form (microtubule t-form cap), as GTP has yet to be cut to GDP.
In vitro microtubule growth is faster at which end?
At the plus (beta-tubulin) end.
What form of free heterodimers are bound to GTP?
The T-form of heterodimers.
What do tubulin subunits (as enzymes) hydrolyze?
Hydrolyze Guanosine triphosphate (GTP).
How do D-form heterodimers occur?
After GTP is hydrolyzed, GDP gets trapped in the tubulin subunits and forms D-form heterodimers.
The microtubule GTP cap stabilizes which end?
The plus end.
What form of heterodimers make up the stabilizing cap?
T-form heterodimers (with GTP).
Why does the GTP cap stabilize at the plus end?
- The plus end favours microtubule growth
2. dimers in t-form bind more strongly to other dimers in the tubule
What does hydrolysis of bound GTP reduce in the subunits?
It reduces binding affinity.
What is dynamic instability?
Dynamic instability is the stoppage of addition of new heterodimers to a microtubule; tubule closure, leads to catastrophe.
What happens when both ends of a microtubule had hydrolyzed, d-form heterodimers?
The tubule is closed, and it leads to catastrophe as heterodimers start to subtract.
Where are microtubules nucleated?
At the MTOC.
The centrosome is an example of what?
A MTOC; nucleates the formation of microtubules.
Plus ends of microtubules radiate where?
Radiate out towards the plasma membrane (PM).
Microtubules are joined to MTOC’s at which end?
At their minus ends.
What do gamma (y) tubulin do at the MTOC?
They are on the centrosome for stabilizing.
What is a gamma-TuRC? (y-TuRC)
A complex of proteins that form a ring structure.
How does gamma-TuRC bind to tubulin subunits?
y-tubulin binds the ring structure of y-TuRC to act as an attachment site for a/B-tubulin dimers.
What do y-tubulin and y-TuRC form?
They form a stabilizing cap at the microtubule minus (alpha) end.
