Functional β-propeller lectins by tandem duplications of repetitive units

Internal symmetry in proteins is likely to be the footprint of evolution by gene duplication and fusion. Like other symmetrical proteins, β-propellers, which are made of 4-10 β-sheet units (blades) circularly arranged around a central tunnel, have probably evolved by duplication and fusion of a rudimentary repetitive unit. However, reproducing the evolution of functional β-propellers by duplication and fusion of repeated units remains a challenge, in particular, because the repeated units must jointly pack to form one hydrophobic core while maintaining intact active sites. As model for generating repeat propellers, we chose tachylectin-2--a highly symmetrical five-bladed β-propeller lectin with five sugar-binding sites. We report the engineering of folded and functional lectins by duplication and fusion of repetitive sequence modules taken from tachylectin-2. The repeated modules comprise three strands of one blade plus one strand of the next blade, thus enabling the closure of the propeller's ring via strand-strand Velcro-like interactions. Duplication and fusion of five modules with the same sequence gave rise to a highly aggregated protein, yet its soluble fraction exhibited lectin function. Subsequently, a library of diversified sequence modules fused in tandem was selected by phage display for glycoprotein binding. A range of new lectins were isolated with binding and biophysical properties that resemble those of wild-type tachylectin-2. These results demonstrate the ability to construct folded and functional globular repeat proteins, and support the role of duplication and fusion of elementary modules in the evolutionary routes that led to the β-propellers fold.