GXXXG and GXXXA motifs stabilize FAD and NAD(P)-binding Rossmann folds through C(alpha)-H... O hydrogen bonds and van der waals interactions.

Here we present evidence that domains in soluble proteins containing either the GXXXG or GXXXA motif are stabilized by the interaction of a beta-strand with the following alpha-helix. As an example, we characterized a beta-strand-helix interaction from the FAD or NAD(P)-binding Rossmann fold. The Rossmann fold is one of the three most highly represented folds in the Protein Data Bank (PDB). A subset of the proteins that adopt the Rossmann fold also bind to nucleotide cofactors such as FAD and NAD(P) and function as oxidoreductases. These Rossmann folds can often be identified by the short amino acid sequence motif, GX(1-2)GXXG. Here, we present evidence that in addition to this sequence motif, Rossmann folds that bind FAD and NAD(P) also typically contain either GXXXG or GXXXA motifs, where the first glycyl residue of these motifs and the third glycyl residue of the GX(1-2)GXXG motif are the same residue. These two motifs appear to stabilize the Rossmann fold: the first glycyl residue of either the GXXXG or GXXXA motif contacts the carbonyl oxygen atom from the first glycyl residue of the GX(1-2)GXXG motif consistent with the formation of a C(alpha)-H cdots, three dots, centered O hydrogen bond. In addition, both the glycyl and alanyl residues of the GXXXG or GXXXA motifs form van der Waals interactions with either a valine or isoleucine residue located either seven or eight residues further back along the polypeptide chain from the first glycine of the GXXXG or GXXXA motifs. Therefore, we combine both the GX(1-2)GXXG and GXXXG/A motifs into an extended motif, V/IXGX(1-2)GXXGXXXG/A, that is more strongly indicative than previously described motifs of Rossmann folds that bind FAD or NAD(P). The V/IXGX(1-2)GXXGXXXG/A motif can be used to search genomic sequence data and to annotate the function of proteins containing the motif as oxidoreductases, including proteins of previously unknown function.