Design of single-layer beta-sheets without a hydrophobic core

The hydrophobic effect is the main thermodynamic driving force in the foldi ng of water-soluble proteins(1,2), Exclusion of nonpolar moieties from aque ous solvent results in the formation of a hydrophobic core in a protein, wh ich has been generally considered essential for specifying and stabilizing the folded structures of proteins(1-6). Outer surface protein A (OspA) from Borrelia burgdorferi contains a three-stranded beta-sheet segment which co nnects two globular domains(7). Although this single-layer beta- sheet segm ent is exposed to solvent on both faces and thus does not contain a hydroph obic core, the segment has a high conformational stability(8), Here we repo rt the engineering of OspA variants that contain larger single-layer beta-s heets (comprising five and seven beta-strands) by duplicating a beta-hairpi n unit within the beta-sheet, Nuclear magnetic resonance and small-angle X- ray scattering analyses reveal that these extended single-layer beta-sheets are formed as designed, and amide hydrogen-deuterium exchange and chemical denaturation show that they are stable. Thus, interactions within the beta -hairpin unit and those between adjacent units, which do not involve the fo rmation of a hydrophobic core, are sufficient to specify and stabilize the single-layer beta-sheet structure. Our results provide an expanded view of protein folding, misfolding and design.