Discovery, Properies And Examples Of Proto-Oncogenes Flashcards
What do we see when we compare the genetic maps of transforming and non transforming retroviruses?
That they are identical for the most part, and encode most of the same proteins but there is one gene difference between them
Which gene is present in transforming retroviruses but absent in non-transforming retroviruses?
The SARC gene - sarc for sarcoma. The tumour causing gene
Which gene of RSV was suspected to cause transformation?
The sarc gene, since other extremely similar retroviruses which are non-transforming, do not express the sarc gene
Describe how the sarc gene was isolated experimentally
We take the wild type viral RNA of RSV.
Reverse transcription works and a cDNA copy of the genome is made, the RNA is destroyed (by an RNAse) and we are left with the cDNA (single stranded) copy of the RSV genome.
We hybridise this stand of DNA with an RNA strand of a tranformation-deficient virus strand without the src gene (e.g. The genome of avain leukosis virus). The two strands will pair by complementary base pairing; The part which is not hybridised and left out is cleaved –> this is the src gene.
The reason the src sequence doesn’t hybridise is because it is missing on the “transformation-deficient” stand
Once a probe that was specific for sarc was obtained, what was done with it experimentally? And what did these findings show?
The DNA of cells who had and had not been infected by RSV was probed with it. It was expected that the sarc probe would hybridize with only the RSV infected cells but instead it hybrise in both cells!
This tells us that even cells that had never seen RSV contained sequences that are closely related to sarc
Therefore tumour viruses don’t transform cells by bringing new genetic information that doesn’t exist into our genome, but the seeds of cancer are within us
Our own cells have a cellular sarc
What is sarc and its function?
Sarc is a non-receptor Tyrosine Kinase. It is a protein involed in cell signalling
Describe the structure of C-sarc
C-sarc is a non-membrane bound protein. Towards the N-terminus it has 2 sarcmology domains, SH2 and SH3, which are involved in protein-protein interactions and downstream signalling.
The SH2 and SH3 domains of the protein are able to interact with a tyrosine residue on the tail of the enzyme; it folds and inactvate the enzyme.
Open protein - active
Folded protein - inactive
Describe the role of the regulatory tyrosine residue on c-sarc
In normal cells, the protein is normally found folded in its inactive conformation, upon stimulus by a signal (usually growth factors) it opens and performs its tyrosine kinase activity - very rapidly the tyrosine residue is PHOSPHORYLATED and the protein returns to its inactive folded conformation. Therefore signalling via c-src is under very strict regulation
What are the differences between v-sarc and c-sarc?
- Level of expression: The promotor regions in v-sarc are very strong and therefore once integrated v-sarc is produced far more than c-sarc
- V-sarc is mutated. The result is a truncated protein which lacks the regulatory tyrosine residue which is essential for switching the protein between active and inactive conformations - v-sarc is always active
If you overexpress c-sarc, is this sufficient to form a tumour?
YES!
How did RSV acquire sarc and how did v-sarc come to be?
In one instance it was carried away and sighltly modified after integration into host genome (in the src region).
It is thought that the v-src gene’s movement away from c-src is not natural selection but simply genetic drift - as there is no advantage for the virus causing cancer - it just happens to do so
Numerous oncogens are transcription factors. True/False?
True
When transcription factors are not regulated they can cause ___________
Cancer
