Contrary to popular belief, bacteria and viruses are not the
only means of transmitting diseases. Certain mammalian diseases, such as sheep
scrapie, Kuru, and mad cow disease, are caused by an infectious protein called
PrP (Prion Protein). A prion is an altered form of a cellular protein that no
longer functions normally. The prion conformation seems to be infectious, and
one model proposes that a prion triggers a chain reaction, causing normal
isoforms of the protein to fold into the prion conformation, progressively
generating prion aggregates that result in a phenotypic change.
At least two proteins, Ure2 and Sup35, have been identified as prions in the
yeast species Saccharomyces cerevisiae, but our research focuses on the Sup35
protein (Sup35p). Sup35, an essential protein, is involved in translation
termination in S. cerevisiae. When the normal Sup35p folds into the less active
prion conformation, translation termination is compromised. This prion form of
Sup35p is known as [PSI+].
Several well known properties of [PSI+] are (1) the curing of [PSI+] by
guanidine hydrochloride (GuHCl) and also by DMSO, and (2) growth inhibition in
a [PSI+] background due to the overproduction of Sup35p.
In order to identify other yeast genes involved in the three phenomena
mentioned above, we plan to mutagenize each yeast gene. This genome-wide screen
involves approximately 6200 genes, and the mutagenesis is carried out by a
bacterial transposon. We are screening for mutants that prevent [PSI+] curing
by GuHCl or DMSO or for mutants that are no longer growth inhibited by
overproducing Sup35p. Analysis of the genes we uncover will allow us to develop
a model that explains the mechanisms behind curing and growth inhibition.