TUTORIAL: Exploitation of the actin cytoskeleton by Listeria Flashcards
Where does the energy for Listeria movement come from?
ATP hydrolysis as new actin monomers bind.
What is Listeria’s mechanism for movement within cytoplasm?
Uses elements of the host cell cytoskeleton.
Comet tails contain actin.
Without motility, bacteria cannot spread through the infected cell efficiently and cannot exit the cell or invade neighbouring cells.
What is Listeria’s mechanism for movement within cytoplasm?
Uses elements of the host cell cytoskeleton.
Comet tails contain actin.
Without motility, bacteria cannot spread through the infected cell efficiently and cannot exit the cell or invade neighbouring cells.
What is the bacterial protein that Listeria motility is dependent upon?
ActA.
What is ActA and what is its relevance to Listeria infection in humans?
Bacterial protein.
Contains 4 proline-rich repeats flanked by amino acids (aspartate, glutamate) which are homologous to those seen in auxin, an endogenous protein in host cells.
Listeria expresses ActA at one pole of the bacterium.
ActA can itself bind actin in the host cell cytoplasm, sufficient to allow limited motility.
What is required for full motility of Listeria bacteria?
VASP binding by the proline-rich region of ActA protein.
10µm/min.
What follows VASP binding in Listeria infection?
Recruitment of Arp complexes which act as nuclei for new actin filaments, increasing the rate of comet tail formation
The preferred ends of the new filaments are oriented towards the bacterium.
VASP also binds profilin to this part of the cell, ensuring a good supply of ATP-containing G-actin molecules for polymerisation.
Where is zyxin found, and what is its role?
In the focal contacts of cells where it is associated with actin stress fibres.
Binds VASP protein, which in turn binds profilin and Arp complex.
What is the pathogenesis of Listeria?
Binds to the outside of cells, via one of the cell surface adhesion molecules (E-cadherin).
This triggers internalisation by normal endocytosis and phagocytosis mechanisms.
Once the bacterium is in an acidic compartment within the cell, a special lysin protein is produced which breaks down the membrane of the organelle and allows Listeria access to the cytoplasm.
It then spreads throughout the cell.
Intracellular motility allows Listeria to move through the cell (in its absence the rate of cell division is reduced) and to migrate from cell to cell or to exit the cell.
Which groups of people are most susceptible to Listeria infection and why?
Any individual with a poor cell-mediated immune response: the very old, very young, pregnant, those undergoing immunosuppression drug therapy, and those suffering from diseases such as AIDS which target the immune system.
During an infection, the vast majority of Listeria will be within host cells where they are not accessible to antibodies or complement. The main form of immune defence is therefore by cell-mediated responses.
What is nucleation of actin?
At least 3 G-actin monomers (each binding ATPor ADP) come together in the right orientation to form a filament nucleus for further elongation.
It is slow unless the nucleus is stabilised, e.g. by a capping protein or Arp complex.
What is elongation of actin?
Actin monomers add to pre-existing filaments at either end.
Equilibrium differs for the 2 filament ends and according to whether actin binds ATP (favours addition) or ADP (favours loss).
ATP hydrolysed to ADP within the filament.
What is the effect of actin binding proteins?
Proteins binding free monomer (e.g. beta-thymosin) decrease polymerisation.
Some (e.g. profilin) enhance the normal rate of replacing ADP with ATP on monomers and favour polymerisation.
Capping proteins bind to one or other filament end and prevent polymerisation there.
Severing proteins cause filaments to split, exposing 2 new ends.
Cross-linking proteins bind filaments together, either at right angles (e.g. filamin) or in parallel (e.g. alpha-actinin)
