Sup35p

Sup35p is the Saccharomyces cerevisiae (a yeast) eukaryotic translation release factor. More specifically, it is the yeast eukaryotic release factor 3 (eRF3), which forms the translation termination complex with eRF1 (Sup45p in yeast). This complex recognizes and catalyzes the release of the nascent polypeptide chain when the ribosome encounters a stop codon. While eRF1 recognizes stop codons, eRF3 facilitates the release of the polypeptide chain through GTP hydrolysis.

Partial loss of function results in nonsense suppression, in which stop codons are ignored and proteins are abnormally synthesized with carboxyl terminal extensions. Complete loss of function is fatal.

Sup35p was shown to propagate in a prion form in 1994 by Reed Wickner. For this reason it is an intensely studied protein. When yeast cells harbor Sup35p in the prion state the resulting phenotype is known as [PSI+]. In [PSI+] cells Sup35p exists in an amyloid state that can be propagated and passed to daughter cells. This results in less soluble and functional protein and thus in an increased rate of nonsense suppression (translational read-through of stop codons).

The overexpression of the gene has been shown to induce the [Psi+] conformation.

Several journal articles have suggested that the ability to interconvert between [PSI+] and [psi-](prion-free) states provides an evolutionary advantage, but this remains an area of much debate.

Susan Lindquist has shown that isogenic populations of yeast can express different phenotypes based on whether they had the prion form of Sup35p or the non-prion form. She did an experiment where seven strains of yeast with different genetic backgrounds were grown under many different stressful conditions, with matched [PSI+] and [psi-] strains. In some cases, the [PSI+] version grew faster, in others [psi-] grew faster. She proposed that [PSI+] may act as an evolutionary capacitor to facilitate adaptation by releasing cryptic genetic variation in natural populations at times of stress. This variation would lie beyond stop codons, which show a high rate of in-frame loss in yeast. Mathematical models suggest that [PSI+] may have evolved for this function.

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