PSI (prion)

[PSI+] was first identified by Cox as a modifier of some types of translational suppression of mutant genes. The initial discovery of [PSI+] was made in a strain auxotrophic for the adenine due to a nonsense mutation Further investigation found that [PSI+] is the misfolded form of Sup35, which is an important factor for translation termination during protein synthesis. It is believed that [PSI+] causes suppression of nonsense mutations by sequestering functional Sup35 in non-functional aggregates, thereby allowing stop codon readthrough. [PIN+], in turn, is the misfolded form of the protein Rnq1. However, the normal function of this protein is unknown to date. It is of note that for the induction of most variants of [PSI+], the presence of [PIN+] is required. Though reasons for this are poorly understood, it is suggested that [PIN+] aggregates may act as “seeds” for the polymerization of [PSI+]. Two modified versions of Sup35 have been created that can induce [PSI+] in the absence of [PIN+] when overexpressed. One version was created by digestion of the gene with BalI, which results in a protein consisting of only the M and N portions of Sup35. The other is a fusion of Sup35NM with HPR, a human membrane receptor protein.

Laboratories commonly identify [PSI+] by growth of a strain auxotrophic for adenine on media lacking adenine, similar to that used by Cox et al. These strains cannot synthesize adenine due to a nonsense mutation in one of the enzymes involved in biosynthetic pathway. When the strain is grown on yeast-extract/dextrose/peptone media (YPD), the blocked pathway results in buildup of a red-colored intermediate compound, which is exported from the cell due to its toxicity. Hence, color is an alternative method of identifying [PSI+] -- [PSI+] strains are white or pinkish in color, and [psi-] strains are red. A third method of identifying [PSI+] is by the presence of Sup35 in the pelleted fraction of cellular lysate.