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Eukaryotic Cell, February 2005, p. 289-297, Vol. 4, No. 2
1535-9778/05/$08.00+0     doi:10.1128/EC.4.2.289-297.2005

Role for Hsp70 Chaperone in Saccharomyces cerevisiae Prion Seed Replication

Youtao Song,1 Yue-xuan Wu,2 Giman Jung,1,{dagger} Yusuf Tutar,1 Evan Eisenberg,2 Lois E. Greene,2 and Daniel C. Masison1*

Laboratory of Biochemistry and Genetics, National Institute of Diabetes Digestive and Kidney Diseases,1 Laboratory of Cell Biology, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland2

Received 26 October 2004/ Accepted 14 December 2004

The Saccharomyces cerevisiae [PSI+] prion is a misfolded form of Sup35p that propagates as self-replicating cytoplasmic aggregates. Replication is believed to occur through breakage of transmissible [PSI+] prion particles, or seeds, into more numerous pieces. In [PSI+] cells, large Sup35p aggregates are formed by coalescence of smaller sodium dodecyl sulfate-insoluble polymers. It is uncertain if polymers or higher-order aggregates or both act as prion seeds. A mutant Hsp70 chaperone, Ssa1-21p, reduces the number of transmissible [PSI+] seeds per cell by 10-fold but the overall amount of aggregated Sup35p by only two- to threefold. This discrepancy could be explained if, in SSA1-21 cells, [PSI+] seeds are larger or more of the aggregated Sup35p does not function as a seed. To visualize differences in aggregate size, we constructed a Sup35-green fluorescent protein (GFP) fusion (NGMC) that has normal Sup35p function and can propagate like [PSI+]. Unlike GFP fusions lacking Sup35p's essential C-terminal domain, NGMC did not form fluorescent foci in log-phase [PSI+] cells. However, using fluorescence recovery after photobleaching and size fractionation techniques, we find evidence that NGMC is aggregated in these cells. Furthermore, the aggregates were larger in SSA1-21 cells, but the size of NGMC polymers was unchanged. Possibly, NGMC aggregates are bigger in SSA1-21 cells because they contain more polymers. Our data suggest that Ssa1-21p interferes with disruption of large Sup35p aggregates, which lack or have limited capacity to function as seed, into polymers that function more efficiently as [PSI+] seeds.

* Corresponding author. Mailing address: Laboratory of Biochemistry and Genetics, National Institute of Diabetes Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892. Phone: (301) 594-1316. Fax: (301) 496-9431. E-mail: masisond{at}helix.nih.gov.

{dagger} Present address: New Frontiers Research Laboratories, Toray Industries, Inc., Kanagawa, Japan.

Eukaryotic Cell, February 2005, p. 289-297, Vol. 4, No. 2
1535-9778/05/$08.00+0     doi:10.1128/EC.4.2.289-297.2005




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