5.1. The Cytoskeleton Flashcards
Microtubules…
25nm in diameter.
Provide a rigid intracellular skeleton for some cells.
They function as tracks that some motor proteins can move along in the cell.
Formed from polymerisation of tubulin monomers.
Are polar structures (one has a positive end, which is capable of rapid growth, and the other has a negative end).
Stabilised by embedding into the centrosome. The microtubules tend to lose their subunits if they are not stabilised.
Centrosomes usually lie next to the nucleus.
Centrosome…
Organelle which serves as a main microtubule organising centre in animal cells.
Creates microtubules for mitosis.
Centrioles have a 9-triplet microtubule assembly.
Several hundred microtubules grow outward from the centrosome at any given time.
Highly dynamic, constantly growing and shrinking in size.
Motor proteins…
Use the microtubule network as a scaffold to position organelles.
Energy from repeated cycles of ATP hydrolysis allow motor proteins to move along microtubules or actin filaments.
Can be called cargo proteins as they move cellular components around.
Different motor proteins move along different filaments in different directions.
Kinesins and dyneins…
Kinesins:
- Move towards the plus end.
- ER have receptors on their surface for this.
- Responsible for organelle movement; transport of RNA and proteins; assembly of cilia and flagella; signalling pathways; mitotic spindle formation and chromosome movement.
Dyneins:
- Move towards the minus end.
- Golgi have receptors on their surface for this.
- Responsible for cytoplasmic dynein and ciliary dynein.
Cilia and flagella…
Common locomotory appendages made of microtubules.
Flagella are longer, usually found between 1-8 quantity.
Cilia are shorter, usually found in their many hundreds.
The microtubules in cilia and flagella are arranged in a 9 + 2 array.
Actin microfilaments…
Made of protein actin.
May exist as single filaments, in bundles or networks.
They help the entire cell or parts of the cell to move.
Involved in cytoplasmic streaming and the formation of pseudopodia.
Can be hijacked by listeria which escapes the phagosome and infects other cells through engulfing.
Myosin microfilaments…
First motor protein to be discovered.
Distinct from kinesins and dyneins.
Move along actin filaments.
Abundant in skeletal muscles.
Other types of myosin are found in non-muscle cells.
All myosin have similar motor units but different adaptor domains which allow them to bind to cell components.
Plasma membranes…
Sense difference on one side of the cell and a transmembrane signal is generated.
Actin cortex is reorganised in a localised area beneath the affected membrane.
The centrosome moves to that part of the cell.
The centrosome repositions internal membrane systems.
Cytotoxic T cells…
Cells of the immune system which kill infected host cells.
Proteins on the infected cells surface are recognised by Kev.
It sends signals to the underlying cortex of T cells, altering the cytoskeleton in several ways.
Proteins associated with actin filaments under the zone of contact.
Centrosomes reorient, moving with its microtubules to the zone of contact.
Microtubules position the Golgi directly under the zone of contact.
This focusses the killing machinery.
Intermediate filaments…
Only found in multicellular organisms.
Relatively tough, ropelike structures first identified in muscle cells.
Particularly prominent in cells subject to mechanical stress such as epithelial cells and muscle cells.
Intermediate in size between actin and myosin filaments.
Stabilise the cell structure and resist tension.
In some cells, intermediate filaments maintain the positions of the nucleus and other organelles in the cell.
Tightly woven basket of intermediate filaments forms a nuclear lamina.
More than 50 different intermediate filament proteins have been identified.
Are elongated fibrous protein molecules. Two coiled dimers form antiparallel tetrameric subunits. They are non-polarised.
Keratins…
Epithelial cells contain over 20 distinct keratins.
At least 8 are specific to the hair and nails.
Vimentin…
Forms polymers of single protein species.
The most widely distributed cytoplasmic intermediate filament protein as it is present in so many cell types.
Keratins and vimentins never co-polymerise but can be found in the same cell.
Mechanism stability…
Increasing evidence that intermediate filament proteins aid in resisting mechanical stress.
In genetic diseases, such as epidermolysis bullosa simplex, mutations in the keratin gene in basal layers of epidermis disrupt keratin networks in these cells.
This makes suffers highly susceptible to mechanical injury and blistering.
