Dissection of protein structure and folding by directed mutagenesis

The lysozyme from bacteriophage T4 is being used as a model system to determine the roles of individual amino acids in the folding and stability of a typical globular protein. One general finding is that the protein is very adaptable, being able to accommodate many potentially destabilizing replacements. In order to determine the importance of 'alpha-helix propensity' in protein stability, different replacements have been made within alpha-helical segments of T4 lysozyme. Several such substitutions of the form Xaa-->Ala increase the stability of the protein, supporting the idea that alanine is a strongly helix-favouring amino acid. It is possible to engineer a protein that has up to ten alanines in succession, yet still folds and has normal activity. This illustrates the redundancy that is present in the amino acid sequence. A number of 'cavity-creating' mutants of the form Leu-->Ala have been constructed to understand better the nature of hydrophobic stabilization. The structural consequences of these mutations differ from site to site. In some cases the protein structure hardly changes at all; in other cases removal of the wild-type side-chain allows surrounding atoms to move in and occupy the vacated space, although a cavity always remains. The destabilization of the protein associated with these cavity-creating mutations also varies from case to case. The results suggest how to reconcile recent conflicting reports concerning the strength of the hydrophobic effect in proteins.