Yeast As A Model Flashcards
Many enzymes are made in the inactive form
What must happen to them to become active?
Sequential modifications must occur
For these modifications to occur, a distinct set of enzymes need to be exposed to proteins.
Revision. What are the major features of membrane trafficking pathways?
As in the exocytic and endocytic pathways? And where can proteins be modified
Secretory pathway (exocytic). This is when material moves from the endoplasmic reticulum to the Gogli, then the endosomes
The endocytic pathway - this bring material into the cell to the endosome then to (lysosomes potentially) then the golgi, then the ER
Proteins can be modified as they move from the ER and the golgi apparatus.
What is retrograde movement in the exocytic secretory pathways?
Movement of material back along the pathway
E.g. if material is moved from the ER to the golgi, then retrograde movement is from the golgi BACK to the ER.
So remember the trafficking pathways through a cell
How did we identify these pathways? And what type of models should we use?
This was done via genetic testing on mice, drosp, zebrafish.
To study membrane trafficking you would want a single celled organism as trafficking is on a simple cellular scale. Hence why we may use yeast.
What is the advantages of using yeast as a model for studying trafficking pathways in the cell?
They can grow as haploid or diploid cells
Entire genome is sequenced
Cheap and easy to grow in large qualities
Have a limited gene diversity. For example there is one gene for actin so even a small mutation in this gene would have an instant effect
Fundamental pathways are conserved between humans and yeast
What are the disadvantages of using yeast as a model for studying trafficking pathways in humans?
Yeast have limited cell to cell contact. So it will be hard to know about multicellularity processes such as exchange of material between cells to be endocytosed
They are very small (5 µm) so high resolution imaging is needed to look at intracellular compartments.
Yeast cells have a cell wall
So thus what processes are yeast experiments likely to tell us about? And what are they useful in telling us?
They will be useless at actually telling us about membrane trafficking
Multicellularity
Cell cycle regulation
Cell wall synthesis
However they are useful in identifying the genes involved in secretory pathways.
Who did experiments to look at the genes in secretory pathways? And what did their experiment say?
Novick and schekman 1980
Their experiment said if proteins couldnt be secreted from the yeast cells ER, then cells were secretory deficient (sec)
These yeast cells ER would increase in density as the vesicles can’t leave the ER which causes the ER to enlarge.
Describe Novick and schekman 1980’s experiement: what were the overall results.
Yeast cells were analysed for their ability to secrete enzymes. Such as invertase and acid phosphatase at different temperatures.
Secretory mutant gene strains were the genes which affected the export active invertase and acid phosphatase out of the yeast cells. In spite of mutants the cells still produce these enzymes
Electron microscopy could show accumulation of would - be secretory vesicles
It was showed that mutations in 23 genes affected protein movement out of ER, affecting the cells ultra structure ( cell becomes fatter)
Novick and schekman 1980’s experiment on yeast highlighted that there were 23 genes related to secretion out of yeast cells ER. What did this mean? And what were these genes called?
This meant that there was atleast 23 gene products required to ensure transport of proteins from the ER to the plasma membrane
These were called sec genes
Why weren’t all the genes for exocytosis out of the ER identified by novick and Schekman?
Remember in the experiment looked at temperature changes and exocytosis
- Thus They only looked at temperature sensitive mutants (sec genes) for movement out of the ER
- note not all mutatated genes will stop exocytosis when there is a temperature change, some will do it anyway
- they also only considered exoctyosis coming out of the ER, thus they didnt look at exocytosis from endosomes for instance to look at the cells overall secretion.
- and genes will overlapping functions - as in redundantly functioning genes would not be identified.
What were the genes called which affected exocytosis?
Sec genes.
What is endocytosis?
This is the process in which the plasma membrane invaginate into the cell resulting in the production of a vesicle that is then able to fuse with endosomes and enter the endo lysosomal membrane system.
In terms of endocytosis, what issues occurs with mutations in trafficking from the endosome to lysosomers?
This affects degradative pathways.
Why is endocytosis important? And also what is important to note about vesicles?
Retrieval of molecules.
Or recycling (as in the materials can be sent to the endosome then back to the plasma membrane to be used.
In endocytosis, vesicles form at the plasma membrane which allows material to be transported to the endosome
Always remember that there are vesicles for transport.
