The role of the N-terminal oligopeptide repeats of the yeast Sup35 prion protein in propagation and transmission of prion variants

The cytoplasmic PSI+] determinant of Saccharomyces cerevisiae is the prion form of the Sup35 protein. Oligopeptide repeats within the Sup35 N-terminal domain (PrD) presumably are required for the stable PSI+] inheritance that in turn involves fragmentation of Sup35 polymers by the chaperone Hsp104. The nonsense suppressor PSI+] phenotype can vary in efficiency probably due to different inheritable Sup35 polymer structures. Here we study the ability of Sup35 mutants with various deletions of the oligopeptide repeats to support PSI+] propagation. We define the minimal region of the Sup35-PrD necessary to support PSI+] as amino acids 1-64, which include the first two repeats, although a longer fragment, 1-83, is required to maintain weak PSI+] variants. Replacement of wild-type Sup35 with deletion mutants decreases the strength of the PSI+] phenotype. However, with one exception, reintroducing the wild-type Sup35 restores the original phenotype. Thus, the specific prion fold defining the PSI+] variant can be preserved by the mutant Sup35 protein despite the change of phenotype. Coexpression of wild-type and mutant Sup35 containing three, two, one, or no oligopeptide repeats causes variant-specific PSI+] elimination. These data suggest that PSI+] variability is primarily defined by differential folding of the Sup35-PrD oligopeptide-repeat region.