Association of HSP70 with endonucleases allows the expression of otherwise silent mutations

A subpopulation of the 70 kDa heat shock protein (HSP70) found within the mitochondria of Saccharomyces cerevisiae functions as a stable binding partner of the endonuclease SceI. We have previously found that the SceI endonuclease monomer recognizes and cleaves a unique, 26 bp sequence in vitro. Dimerization with HSP70 changes the specificity of SceI, allowing it to cleave at multiple sequences. This study shows that SuvI, an ortholog of SceI isolated from a different yeast strain, contains two amino acid substitutions, yet it shows the same uni-site specificity in its monomeric form. Binding of HSP70 to the SuvI monomer confers multi-site specificity that is different from that exhibited by the HSP70/SceI heterodimer. Mutation of single residues of SceI to the corresponding residue in SuvI provides enzymes with specificities intermediate between SceI and SuvI when complexed with HSP70. These results suggest that HSP70 interaction with certain endonucleases allows the expression of otherwise silent mutations in them, causing a change in enzyme cleavage specificity.