Higher-order packing interactions in triple and quadruple mutants of staphylococcal nuclease.

Sixty-four triple and 32 quadruple mutants were constructed in the core of staphylococcal nuclease. This is the first time that a large number of multiple mutants with all possible variations and all possible lower-order mutants has been systematically constructed in any protein core. Stabilities were determined by solvent denaturation. The energetic effects of these multiple mutants have been analyzed in combination with the stability data from the component single and double mutants. It was found that most of the stability changes in triple and quadruple mutants cannot be correctly predicted from stability effects of component single mutants. However, if the interaction energy between pairs of side chains in the component double mutants is taken into account, correct stability prediction can be made for most triple and quadruple mutants. The data further show that while packing interactions unique to triple and quadruple mutations do occur, they are of much less energetic significance than interactions between pairs of residues. The results presented here show that the packing of a protein interior can be closely approximated in most cases as a series of short-range, nearest-neighbor interactions. This has profound implications for rational protein design and structure prediction.