Primary sequence independence for prion formation

Many proteins can adopt self-propagating beta-sheet-rich structures, termed amyloid fibrils. The URE3 and PSI(+) prions of Saccharomyces cerevisiae are infectious amyloid forms of the proteins Ure2p and Sup35p, respectively. Ure2p forms prions primarily as a result of its sequence composition, as versions of Ure2p with the prion domain amino acids shuffled are still able to form prions. Here we show that prion induction by both Ure2p and Ure2-21p, one of the scrambled versions of Ure2p, is clearly dependent on the length of the inducing fragment. For Ure2-21p, no single sequence is found in all of the inducing fragments, highlighting the sequence independence of prion formation. Furthermore, the sequence of the Sup35p prion domain can also be randomized without blocking prion formation. Indeed, a single shuffled sequence could give rise to several prion variants. These results suggest that PSI(+) formation is driven primarily by the amino acid composition of the Sup35p prion domain, and that the Sup35p oligopeptide repeats are not required for prion maintenance.