Prion-Dependent Switching between Respiratory Competence and Deficiency in the Yeast nam9-1Mutant

Abstract

Nam9p is a protein of the mitochondrial ribosome. The respiration-deficient Saccharomyces cerevisiae strain MB43-nam9-1 expresses Nam9-1p containing the point mutation S82L. Respiratory deficiency correlates with a decrease in the steady level of some mitochondrially encoded proteins and the complete lack of mitochondrially encoded cytochrome oxidase subunit 2 (Cox2). De novo synthesis of Cox2 in MB43-nam9-1 is unaffected, indicating that newly synthesized Cox2 is rapidly degraded. Respiratory deficiency of MB43-nam9-1 is overcome by transient overexpression of HSP104, by deletion of HSP104, by transient exposure to guanidine hydrochloride, and by expression of the C-terminal portion of Sup35, indicating an involvement of the yeast prion [PSI⁺]. Respiratory deficiency of MB43-nam9-1 can be reinduced by transfer of cytosol from S. cerevisiae that harbors [PSI⁺]. We conclude that nam9-1causes respiratory deficiency only in combination with the cytosolic prion [PSI⁺], presenting the first example of a synthetic effect between cytosolic [PSI⁺] and a mutant mitochondrial protein.

ACKNOWLEDGMENTS

We are indebted to G. Schatz for giving A.C. the opportunity to work in his laboratory in the Biozentrum, for generous support, and for fruitful discussions throughout the project. We thank B. Szczesniak and R. Looser for excellent technical assistance. The pEMBLyex4-SUP35 and pEMBLyex4-3ATG plasmids and the OT72 and OT74 yeast strains were kind gifts of M. D. Ter-Avanesyan and Y. O. Chernoff. We thank U. Fünfschilling for help with the yeast translation extracts and B. Ono for strain BO512-4C. Critical reading of the manuscript by A. Paszewski, Y. Dubaquié, S. Merchant, C. Suzuki, and T. Lithgow is gratefully acknowledged.

This study was supported by State Committee for Scientific Research (KBN) grant 6PO4B02915 for A.C. and M.B., KBN grant 6PO4A03312 for A.C., Swiss National Science Foundation (SNSF) grant 7IP 051663 covering the costs of A.C.'s stay and research in the Biozentrum, University of Basel, and SNSF grant 3100-050954 to S.R.