Determining Amino Acid Propensity for Propagation of the Prion Sup35 in \u3cem\u3eSaccharomyces cerevisiae\u3c/em\u3e

Proteins are built based on a DNA sequence template and fold into distinctive shapes or conformations. Amyloids are beta sheet-rich proteins that can fold into two stable states, a normal form and an abnormal form. Prions, the infectious amyloids, are present in many eukaryotes, including humans, cows, sheep, and the baker’s yeast, Saccharomyces cerevisiae, and can cause the conversion of native protein structure into amyloid form. Unusually, yeast prion proteins have been shown to form amyloid structure on the basis of amino acid composition, independent of each protein’s primary structure. The most well-studied prion protein in yeast, Sup35, consists of a prion-forming domain, a middle domain, and a functional domain. Within the prion-forming domain there is a glutamine/asparagine-rich (Q/N-rich) tract and a region of 5 ½ degenerate oligopeptide repeats (the oligopeptide repeat domain, or ORD). The Q/N tract has been previously implicated in prion nucleation and the ORD was shown to be necessary for prion propagation. In this study, we generated a library of yeast cells carrying varying, randomly-generated, mutations in the fourth repeat of the Sup35 ORD region, and on this basis we have been able to assign propensity values for each amino acid to favor or disfavor prion propagation