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Vol. 19, Issue 6, 2433-2443, June 2008

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Variant-specific [PSI⁺] Infection Is Transmitted by Sup35 Polymers within [PSI⁺] Aggregates with Heterogeneous Protein Composition

Department of Biological Sciences, University of Illinois at Chicago, Chicago, IL 60607

Submitted January 25, 2008; Revised February 20, 2008; Accepted March 10, 2008
Monitoring Editor: Jonathan Weissman

The [PSI⁺] prion is the aggregated self-propagating form of the Sup35 protein from the yeast Saccharomyces cerevisiae. Aggregates of Sup35 in [PSI⁺] cells exist in different heritable conformations, called "variants," and they are composed of detergent-resistant Sup35 polymers, which may be closely associated with themselves, other proteins, or both. Here, we report that disassembly of the aggregates into individual Sup35 polymers and non-Sup35 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 seem 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 that it interacts poorly with the nonaggregated Sup35 found in [psi^–] cells. Hsp104, Sis1, and Sse1 interact preferentially with the prion versus nonprion form of Sup35, whereas Sla2 and Ssb1/2 interact with both forms of Sup35 with similar efficiency.

Address correspondence to: Susan W. Liebman (sueL{at}uic.edu)

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