[PSI+] and [URE3]

In 1965, Brian Cox, a geneticist working with the yeast Saccharomyces cerevisiae, described a genetic trait (termed [PSI+]) with an unusual pattern of inheritance. The initial discovery of [PSI+] was made in a strain auxotrophic for adenine due to a nonsense mutation.^[6] Despite many years of effort, Cox could not identify a conventional mutation that was responsible for the [PSI+] trait. In 1994, yeast geneticist Reed Wickner correctly hypothesized that [PSI+] as well as another mysterious heritable trait, [URE3], resulted from prion forms of the normal cellular proteins, Sup35p and Ure2p, respectively.^[7] The names of yeast prions are frequently placed within brackets to indicate that they are non-mendelian in their passage to progeny cells, much like plasmid and mitochondrial DNA.^{[citation needed]}

Further investigation found that [PSI+] is the result of a self-propagating misfolded form of Sup35p (a 201 amino acid long protein), which is an important factor for translation termination during protein synthesis.^[8] In [PSI+] yeast cells the Sup35 protein forms filamentous aggregates known as amyloid. The amyloid conformation is self-propagating and represents the prion state. Amazingly distinct prion states exist for the Sup35 protein with distinct properties and these distinctions are self-propagating.^[9] Other prions also can form distinct different variants (or strains).^[10] It is believed that suppression of nonsense mutations in [PSI+] cells is due to a reduced amount of functional Sup35 because much of the protein is in the amyloid state. The Sup35 protein assembles into amyloid via an amino-terminal prion domain. The structure is based on the stacking of the prion domains in an in-register and parallel beta sheet conformation.^[11]

An important finding by Chernoff, in a collaboration between the Liebman and Lindquist laboratories, was that a protein chaperone was required for [PSI+] to be maintained.^[12] Because the only function of chaperones is to help proteins fold properly, this finding strongly supported Wickner's hypothesis that [PSI+] was a heritable protein state (i.e. a prion). Likewise, this finding also provided evidence for the general hypothesis that prions, including the originally proposed mammalian PrP prion, are heritable forms of protein. Because of the action of chaperones, especially Hsp104, proteins that code for [PSI+] and [URE3] can convert from non-prion to prion forms. For this reason, yeast prions are good models for studying factors like chaperones that affect protein aggregation.^[10] Also, the IPOD is the sub-cellular site to which amyloidogenic proteins are sequestered in yeast, and where prions like [PSI+] may undergo maturation.^[13] Thus, prions also serve as substrates to understand the intracellular processing of protein aggregates such as amyloid.^{[citation needed]}

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 the 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.

When exposed to certain adverse conditions, in some genetic backgrounds [PSI+] cells actually fare better than their prion-free siblings;^[14] this finding suggests that the ability to adopt a [PSI+] prion form may result from positive evolutionary selection.^[15] It has been speculated that the ability to convert between prion-infected and prion-free forms acts as an evolutionary capacitor to enable yeast to quickly and reversibly adapt in variable environments. Nevertheless, Reed Wickner maintains that [URE3] and [PSI+] are diseases,^[16] although this claim has been challenged using theoretical population genetic models.^[17]

[PIN+] / [RNQ+]

The term [PIN+] was coined by Liebman and colleagues from Psi-INducibility, to describe a genetic requirement for the formation of the [PSI+] prion.^[18] They showed that [PIN+] was required for the induction of most variants of the [PSI+] prion. Later they identified [PIN+] as the prion form of the RNQ1 protein ^[19][20][21] The more precise name [RNQ+] is now sometimes used because other factors or prions can also have a Psi-inducing phenotype.^{[citation needed]}

A non-prion function of Rnq1 has not been definitively characterized. Though reasons for this are poorly understood, it is suggested that [PIN+] aggregates may act as "seeds" for the polymerization of [PSI+] and other prions.^[22][23][24] The basis of the [PIN+] prion is an amyloid form of Rnq1 arranged in in-register parallel beta sheets, like the amyloid form of Sup35.^[25] Due to similar amyloid structures, the [PIN+] prion may facilitate the formation of [PSI+] through a templating mechanism.^{[citation needed]}

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 the restriction enzyme Bal2, which results in a protein consisting of only the M and N portions of Sup35.^[26] The other is a fusion of Sup35NM with HPR, a human membrane receptor protein.^{[citation needed]}