Three-dimensional profiles for analysing protein sequence-structure relationships.

In the method of 3D (three-dimensional) profiles, each residue position in a protein is characterized by its environment and is represented by a row of 20 numbers in a table, the profile. These numbers are the statistical preferences (called 3D-1D scores) of each of the 20 amino acids for this environment. A profile is computed from the coordinates of a protein model, and it gives a score S for any amino acid sequence folded as the model. To date 3D profiles have found three applications. The first is to identify other protein sequences which are folded in the same general pattern as the structure from which the profile was prepared. These are sequences which have high scores for the profile computed from the model. The second is to assess the validity of protein models, however determined. Correct models are found to give profiles that have high scores for their own amino acid sequences, and incorrect models are found to have lower scores. The example of the X-ray structure determination of diphtheria toxin is discussed. The third application is to assess which is the stable oligomeric state of a folded protein. Several examples suggest that the highest profile score for a sequence is achieved when the protein is aggregated into its most stable oligomeric state.