Heterologous Prion Interactions Are Altered by Mutations in the Prion Protein Rnq1p

Abstract

Prions in the yeast Saccharomyces cerevisiae show a surprising degree of interdependence. Specifically, the rate of appearance of the [ PSI+] prion, which is thought to be an important mechanism to respond to changing environmental conditions, is greatly increased by another prion, [ RNQ+]. While the domains of the Rnq1 protein important for formation of the [ RNQ+] prion have been defined, the specific residues required remain unknown. Furthermore, residues in Rnq1p that mediate the interaction between [ PSI+] and [ RNQ+] are unknown. To identify residues important for prion protein interactions, we created a mutant library of Rnq1p clones in the context of a chimera that serves as proxy for [ RNQ+] aggregates. Several of the mutant Rnq1p proteins showed structural differences in the aggregates they formed, as revealed by semi-denaturing detergent agarose gel electrophoresis. Additionally, several of the mutants showed a striking defect in the ability to promote [ PSI+] induction. These data indicate that the mutants formed strain variants of [ RNQ+]. By dissecting the mutations in the isolated clones, we found five single mutations that caused [ PSI+] induction defects, S223P, F184S, Q239R, N297S, and Q298R. These are the first specific mutations characterized in Rnq1p that alter [ PSI+] induction. Additionally, we have identified a region important for the propagation of certain strain variants of [ RNQ+]. Deletion of this region (amino acids 284–317) affected propagation of the high variant but not medium or low [ RNQ+] strain variants. Furthermore, when the low [ RNQ+] strain variant was propagated by Δ284–317, [ PSI+] induction was greatly increased. These data suggest that this region is important in defining the structure of the [ RNQ+] strain variants. These data are consistent with a model of [ PSI+] induction caused by physical interactions between Rnq1p and Sup35p.