The Role of Backbone Flexibility in the Accommodation of Variants That Repack the Core of T4 Lysozyme

To understand better how the packing of side chains within the core influences protein structure and stability, the crystal structures were determined for eight variants of T4 lysozyme, each of which contains three to five substitutions at adjacent interior sites. Concerted main-chain and side-chain displacements, with movements of helical segments as large as 0.8 angstrom, were observed. In contrast, the angular conformations of the mutated side chains tended to remain unchanged, with torsion angles within 20^circ of those in the wild-type structure. These observations suggest that not only the rotation of side chains but also movements of the main chain must be considered in the evaluation of which amino acid sequences are compatible with a given protein fold.