Generating new prions by targeted mutation or segment duplication.

Yeasts contain various protein-based genetic elements, termed prions, that result from the structural conversion of proteins into self-propagating amyloid forms. Most yeast prion proteins contain glutamine/asparagine (Q/N)-rich prion domains that drive prion activity. Here, we explore two mechanisms by which new prion domains could evolve. First, it has been proposed that mutation and natural selection will tend to result in proteins with aggregation propensities just low enough to function under physiological conditions and thus that a small number of mutations are often sufficient to cause aggregation. We hypothesized that if the ability to form prion aggregates was a sufficiently generic feature of Q/N-rich domains, many nonprion Q/N-rich domains might similarly have aggregation propensities on the edge of prion formation. Indeed, we tested four yeast Q/N-rich domains that had no detectable aggregation activity; in each case, a small number of rationally designed mutations were sufficient to cause the proteins to aggregate and, for two of the domains, to create prion activity. Second, oligopeptide repeats are found in multiple prion proteins, and expansion of these repeats increases prion activity. However, it is unclear whether the effects of repeat expansion are unique to these specific sequences or are a generic result of adding additional aggregation-prone segments into a protein domain. We found that within nonprion Q/N-rich domains, repeating aggregation-prone segments in tandem was sufficient to create prion activity. Duplication of DNA elements is a common source of genetic variation and may provide a simple mechanism to rapidly evolve prion activity.