Micelle-Triggered [beta]-Hairpin to [alpha]-Helix Transition in a 14-Residue Peptide from a Choline-Binding Repeat of the Pneumococcal Autolysin LytA

Choline-binding modules (CBMs) have a [beta][beta]-solenoid structure composed of choline-binding repeats (CBR), which consist of a [beta]-hairpin followed by a short linker. To find minimal peptides that are able to maintain the CBR native structure and to evaluate their remaining choline-binding ability, we have analysed the third [beta]-hairpin of the CBM from the pneumococcal LytA autolysin. Circular dichroism and NMR data reveal that this peptide forms a highly stable native-like [beta]-hairpin both in aqueous solution and in the presence of trifluoroethanol, but, strikingly, the peptide structure is a stable amphipathic [alpha]-helix in both zwitterionic (dodecylphosphocholine) and anionic (sodium dodecylsulfate) detergent micelles, as well as in small unilamellar vesicles. This [beta]-hairpin to [alpha]-helix conversion is reversible. Given that the [beta]-hairpin and [alpha]-helix differ greatly in the distribution of hydrophobic and hydrophilic side chains, we propose that the amphipathicity is a requirement for a peptide structure to interact and to be stable in micelles or lipid vesicles. To our knowledge, this "chameleonic" behaviour is the only described case of a micelle-induced structural transition between two ordered peptide structures.