A backbone-reversed all-beta polypeptide (retro-CspA) folds and assembles into amyloid nanofibres

The backbone-reversed or 'retro', form of a model all-beta-sheet protein, Escherichia coli CspA, was produced from a synthetic gene in E.coli in fusion with an N-terminal affinity tag. Following purification under denaturing conditions and dialysis-based removal of urea, the protein was found to fold into a soluble, poorly structured multimer. Upon concentration, this state readily transformed into amyloid nanofibres. Congo Red-binding amorphous forms were also observed. Since a beta-sheet-forming sequence is expected to retain high beta-sheet-forming propensity even after backbone reversal and given the fact that folding of retro-CspA occurs only to a poorly structured form, we conclude that the increase effected in protein concentration may be responsible for the formation of intermolecular beta-sheets, facilitating the bleeding away of the protein's conformational equilibrium into aggregates that generate well-formed fibres. Since every molecule in these fibres contains a peptide tag for binding Ni(2+), the fibres may provide a template for deposition of nickel to generate novel materials.