Amino acid distribution rules predict protein fold: protein grammar for beta-strand sandwich-like structures

Sequence Pattern for Supersecondary Structure of Sandwich-Like Proteins

The goal is to define sequence characteristics of beta-sandwich proteins that are unique for the beta-sandwich supersecondary structure (SSS). Finding of the conserved residues that are critical for protein structure can often be accomplished with homology methods, but these methods are not always adequate as residues with similar structural role do not always occupy the same position as determined by sequence alignment. In this paper, we show how to identify residues that play the same structural role in the different proteins of the same SSS, even when these residue positions cannot be aligned with sequence alignment methods. The SSS characteristics are (a) a set of positions in each strand that are involved in the formation of a hydrophobic core, residue content, and correlations of residues at these key positions, (b) maximum allowable number of "low-frequency residues" for each strand, (c) minimum allowed number of "high-frequency" residues for each loop, and (d) minimum and maximum lengths of each loop. These sequence characteristics are referred to as "sequence pattern" for their respective SSS. The high specificity and sensitivity for a particular SSS are confirmed by applying this pattern to all protein structures in the SCOP data bank. We present here the pattern for one of the most common SSS of beta-sandwich proteins.

Secondary and Supersecondary Structure of Proteins in Light of the Structure of Hydrophobic Cores

The traditional classification of protein structures (with regard to their supersecondary and tertiary conformation) is based on an assessment of conformational similarities between various polypeptide chains and particularly on the presence of specific secondary structural motifs. Mutual relations between secondary folds determine the overall shape of the protein and may be used to assign proteins to specific families (such as the immunoglobulin-like family). An alternative means of conducting structural assessment focuses on the structure of the protein's hydrophobic core. In this case, the protein is treated as a quasi-micelle, which exposes hydrophilic residues on its surface while internalizing hydrophobic residues. The accordance between the actual distribution of hydrophobicity in a protein and its corresponding theoretical ("idealized") distribution can be determined quantitatively, which, in turn, enables comparative analysis of structures regarded as geometrically similar (as well as geometrically divergent structures which are nevertheless regarded as similar in the sense of the fuzzy oil drop model). In this scope, the protein may be compared to an "intelligent micelle," where local disorder is often intentional and related to biological function-unlike traditional surfactant micelles which remain highly symmetrical throughout and do not carry any encoded information.