TOWARD A SIMPLIFICATION OF THE PROTEIN FOLDING PROBLEM - A STABILIZING POLYALANINE ALPHA-HELIX ENGINEERED IN T4 LYSOZYME

In an attempt to simplify the protein folding problem, and also to further investigate the role of alanine as a helix-stabilizing residue, a series of alanines was introduced within the alpha-helix that includes residues 126-134 of T4 lysozyme. In wild-type lysozyme this alpha-helix contains alanine residues at positions 129, 130, and 134. Mutant lysozymes with alanines substituted at positions 128, 131, 132, and 133, either as single substitutions or in selected combinations, were constructed by oligonucleotide-directed mutagenesis. With the exception of the replacement of Leu 133, which is buried within the hydrophobic core of the protein, all the variants were more stable than wild-type lysozyme. The variant with alanines substituted at positions 128, 131, and 132 (E128A/V131A/N132A), which incorporates the sequence Ala 128-Ala 129-Ala 130-Ala 131-Ala 132-Leu 133-Ala 134, has a melting temperature 3.3-degrees-C above that of wild-type lysozyme. Determination of the crystal structure of this mutant lysozyme shows that the replacement of Glu 128, Val 131, and Asn 132 with alanine causes alpha-helix 126-134 to rotate 3.4-degrees about an axis parallel to its own axis. This rotation seems to be triggered primarily by the loss of a hydrogen bond between Asn 132 and Ser 117 and is associated with the repacking of several side chains at the interface between alpha-helix 126-134 and the adjacent alpha-helix 115-122. Presumably because of these conformational adjustments, there is a synergistic effect such that the stabilization arising from the triple substitution E128A/V131A/N132A is slightly greater than the sum of the stabilization energies due to the constituent single plus double substitutions E128A and V131A/N132A. The results show that at least for one alpha-helix in T4 lysozyme the replacement of solvent-exposed residues with alanines provides a method to increase the stability of the protein. Also, the finding that a series of alanines can be introduced into T4 lysozyme shows that this might be a way to simplify the protein folding problem.