Replacement of Staphylococcal Nuclease Hydrophobic Core Residues with Those from Thermophilic Homologues Indicates Packing is Improved in Some Thermostable Proteins

The importance of tight hydrophobic core packing in stabilizing proteins found in thermophilic organisms has been vigorously disputed. Here, portions of the cores found in three thermophilic homologues were transplanted into the core of staphylococcal nuclease, a protein of modest stability. Packing of the core was evaluated by comparing interaction energy of the three mutants to the comprehensive mutant library built up previously at these same sites in staphylococcal nuclease. It was found that the interaction energy of one thermophilic sequence is extraordinarily favorable and the interaction energies of other two transplanted thermophilic sequences are good, comparable to the interaction energies of mutant cores based on cores found in mesophilic homologues. As expected when transferring just a portion of the core sequence, the mutant proteins were destabilized overall relative to wild-type staphylococcal nuclease. The overall conclusion is that improvement of packing interactions is a mechanism to confer stability employed in some proteins from thermophiles, but not all.