Combining Thermostable Mutations Increases the Stability of λ Repressor

We have combined three mutations previously shown to stabilize λ repressor against thermal denaturation. Two of these mutations are in helix 3, where Gly-46 and Gly-48 have been replaced by alanines [Hecht, M. H., et al. (1986) Proteins: Struct., Funct., Genet. 1, 43–46]. The other mutation, which replaces Tyr-88 with cysteine, allows the protein to form an intersubunit disulfide bond [Sauer, R. T., et al. (1986) Biochemistry 25, 5992–5998], Calorimetric measurements show that the two alanine substitutions stabilize repressor by about 8 °C, that the disulfide bond stabilizes repressor by about 8 °C, and that the triple mutant is 16 °C more stable than wild-type repressor.