Background: The kinase Tor is the target of the immunosuppressive drug rapamycin and is a member of the phosphatidylinositol kinase (PIK)-related kinase family. It plays an essential role in progression through the G1 phase of the cell cycle. The molecular details of Tor signaling remain obscure, however.

Results: We isolated two Saccharomyces cerevisiae genes, BMH1 and BMH2, as multicopy suppressors of the growth-inhibitory phenotype caused by rapamycin in budding yeast. BMH1 and BMH2 encode homologs of the 14-3-3 signal transduction proteins. Deletion of one or both BMH genes caused hypersensitivity to rapamycin in a manner that was dependent on gene dosage. In addition, alterations in the phosphopeptide-binding pocket of the 14-3-3 proteins had dramatically different effects on their ability to relieve the growth-arresting rapamycin phenotype. Mutations that prevented 14-3-3 from binding to a phosphoserine motif abolished its ability to confer rapamycin resistance. In contrast, substitution of two residues in 14-3-3 that surround these phosphoserine-binding sites conferred a dominant rapamycin-resistant phenotype.

Conclusions: Our studies reveal 14-3-3 as an important component in rapamycin-sensitive signaling and provide significant new insights into the structure and function of 14-3-3 proteins.