Structures of single-layer ß-sheet proteins evolved from ß-hairpin repeats.

Free-standing single-layer ß-sheets are extremely rare in naturally occurring proteins, even though ß-sheet motifs are ubiquitous. Here we report the crystal structures of three homologous, single-layer, anti-parallel ß-sheet proteins, comprised of three or four twisted ß-hairpin repeats. The structures reveal that, in addition to the hydrogen bond network characteristic of ß-sheets, additional hydrophobic interactions mediated by small clusters of residues adjacent to the turns likely play a significant role in the structural stability and compensate for the lack of a compact hydrophobic core. These structures enabled identification of a family of secreted proteins that are broadly distributed in bacteria from the human gut microbiome and are putatively involved in the metabolism of complex carbohydrates. A conserved surface patch, rich in solvent-exposed tyrosine residues, was identified on the concave surface of the ß-sheet. These new modular single-layer ß-sheet proteins may serve as a new model system for studying folding and design of ß-rich proteins.