Identification of an N-capping box that affects the alpha 6-helix propensity in glutathione S-transferase superfamily proteins: a role for an invariant aspartic residue.

We have identified an N-capping box motif (Ser/Thr-Xaa-Xaa-Asp) that is strictly conserved, at the beginning of alpha 6 helix, in all glutathione S-transferases (GSTs) and most of the related superfamily proteins. By using CD and peptide modelling we have demonstrated that the capping box residues have an important role in determining the helical conformation adopted by this fragment in the hydrophobic environment of the protein. This is an example in which a local motif, contributing to nucleation of a structural element essential to the global folding of the protein, is strictly conserved in a superfamily of homologous proteins.