The evolutionary conserved Tel2 protein appears to function as a co-chaperone required for the activity of phosphatidylinositol 3-kinase-like protein kinases (PIKKs). Since Saccharomyces cerevisiae Tel2 is essential for viability and only a single point mutant (Tel2-1) had been characterized so far, the possible range of phenotypes associated with Tel2 mutations was unknown. We used random in vitro mutagenesis and plasmid shuffling to create additional point mutants. No significant sensitivity towards DNA damaging agents or hydroxyurea was evident, indicating that Tel2 is not required for Mec1 function. However, as frequent novel phenotypes, we detected slow growth or enhanced lethality in response to rapamycin that could be correlated with lower level and activity of Tor1 or of both Tor1 and Tor2, respectively. The newly isolated mutant with the most severe phenotype, Tel2-13, is comprised of 8 amino acid changes. Two mutated residues of Tel2-13 near the N-terminus and close to Tel2-1 are sufficient for shortened telomeres whereas multiple mutations within the C-terminal two thirds of the protein are required for enhanced rapamycin lethality. Our findings demonstrate separation of function explainable by differential binding of Tel2 to its PUCK substrates Tell or Tor1/Tor2 and thus a critical contribution of Tel2 to the interface that links various PIKKs to this chaperone complex. (C) 2011 Elsevier Inc. All rights reserved.