lecture 6 - The cytoskeleton Flashcards
The cytoskeleton
- Made of long polymeric proteins
- Found in the cytoplasm and between organelles (not inside organelles)
- Serves many different functions
Cytoskeleton in the neuron
Cytoskeleton in cell motility
Cytoskeleton in cell division
Three types of cytoskeletal
Microfilaments
Microtubules
Intermediate filaments
Microfilaments
Made of actin monomers
* Thin and stiff, can be built and destroyed quickly
* Reinforce and shape the cell membrane
Microtubules
Made of tubulin monomers
* Very strong, have a clear polarity (more on this later)
* Generally used to move things inside the cell
Intermediate filaments
- Made of different types of monomers
- Flexible, can withstand mechanical stress
- Generally used to hold things in place
Actin
Each monomer is a single
polypeptide, which exists in the
cytoplasm as a soluble protein.
Each monomer binds exactly one
ATP or ADP. Actin+ATP
polymerises quickly, but
Actin+ADP depolimerises.
Each monomer is also called
globular actin (G-actin). The
filaments are called filamentous
actin (F-actin)
Microfilaments
The two ends of the filament are different from each other.
1. 2. 3. Actin+ATP binds the filament at the “plus” end
Actin hydrolises the bound ATP to ADP while in the filament
Actin+ADP leaves the filament at the “minus” end
Each monomer travels along the filament over time. This
movement is called treadmilling.
Arp2/3 makes filaments spread
Because Arp2/3 caps the minus end of the filament, filaments
grow at the plus end but do not shrink at the minus end.
Many membrane receptors mediate cell movement (taxis) by
activating the Arp2/3 complex. This causes the cell as a whole
to stretch in that direction (leading edge).
The back end of the cell (trailing edge) does not have taxis
signals, Arp2/3 detaches, and the actin mesh falls apart. The
cell as a whole shrinks from that direction.
Formin makes filaments extend
Formin increases the rate of polymerisation at the plus end by facilitating the addition of new monomers.
This causes many filaments to quickly grow in the same direction.
Some membrane receptors bind to formin to make the cell extend in that direction.
Actin structures often coexist
Tubulin
Each subunit is a dimer of two polypeptides (α and β).
α-tubulin binds one GTP, and never exchanges it. β-tubulin binds either one GTP or one GDP.
A microtubule is made up of 13x protofilaments.
Microtubules
Most microtubules in the cell are capped at the minus end.
At the plus end:
GTP-tubulin binds and extends the tubule
GTP is hydrolysed to GDP while in the tubule
When GTP-tubulin runs out, GDP-tubulin leaves the tubule
The microtubule grows and shrinks from the same end. This is called dynamic instability.
The centrosome
An important organelle in animal cells
Acts as a micrutubule organising centre (MTOC). Most microtubules in the cell originates from it.
G1 cells have one and only one. It duplicates in S phase, and G2 cells have two very close together.
It helps position organelles during interphase, and chromosomes during mitosis.
Different MTOCs mediate different cell shapes
The microtubule network positions key organelles
In the figure above:
The microtubule network (green) extends through the cytoplasm and around the nucleus.
The Golgi Apparatus (yellow) is pulled by dynein towards the MTOC. This is how it stays close to the nucleus.
The Endoplasmic Reticulum (blue) is pulled by kinesins away from the MTOC. This is how it spreads through the cytoplasm.
Microtubules also direct vesicles to destination
Axons and dendrites contain microtubule “tracks” that do not come from the centrosome (acentrosomal).
Neurotransmitter vesicles, mitochondria and other components produced in the cell body are trafficked all the way to the axon/dendrite terminus.
Waste products to be recycled and many signalling molecules (e.g. growth factors) are trafficked from the terminus to the cell body.
The tau protein stabilises the axon
Tau (tubulin associated unit) is a microtubule-associated protein (MAP).
It binds microtubules and extends from them with a short arm.
This makes the microtubule stronger, and also helps spacing out thick arrays of microtubules.
Disruptions in tau are a possible cause of Alzheimer’s disease: the tau proteins stick to each other rather than the microtubules, causing the disassembly of the axonal microtubules and the accumulation of tau aggregates (neurofibrillary tangles).
Intermediate filaments
Common structure
Nuclear lamins
Unphosphorylated lamins are the intermediate filaments forming the nuclear lamina.
The lamina surrounds the chromosomes and serves as an anchor for the nuclear envelope.
When the cell undergoes mitosis, the active Cdks phosphorylate the lamins, causing the nuclear envelope to fall apart.
During telophase, the lamins are dephosphorylated and the nuclear envelope reassembles.
Neurofilaments
