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Submitted on January 25, 2008
Revised on February 20, 2008
Accepted on March 10, 2008
Department of Biological Sciences, University of Illinois at Chicago, Chicago, IL, 60607
Monitoring Editor: Jonathan Weissman
The [PSI+] prion is the aggregated self-propagating form of the Sup35 protein from the yeast S. cerevisiae. Aggregates of Sup35 in [PSI+] cells exist in different heritable conformations, called variants, and are composed of detergent-resistant Sup35 polymers, which may be closely associated with themselves and/or other proteins. Here we report that disassembly of the aggregates into individual Sup35 polymers and nonSup35 components increases their infectivity while retaining their variant-specificity, showing that variant-specific [PSI+] infection can be transmitted by Sup35 polymers alone. Morphological analysis revealed that Sup35 isolated from [PSI+] yeast has the appearance of short barrels, and bundles, which appear to be composed of barrels. We show that the major components of two different variants of [PSI+] are interacting infectious Sup35 polymers and Ssa1/2. Using a candidate approach, we detected Hsp104, Ssb1/2, Sis1, Sse1, Ydj1, and Sla2 among minor components of the aggregates. We demonstrate that Ssa1/2 efficiently binds to the prion domain of Sup35 in [PSI+] cells, but interacts poorly with the nonaggregated Sup35 found in [psi-] cells. Hsp104, Sis1, and Sse1 interact preferentially with the prion versus nonprion form of Sup35, while Sla2 and Ssb1/2 interact with both forms of Sup35 with similar efficiency.