LAUSR

The anionic surfactant sodium dodecyl sulfate (SDS) interacts strongly with most globular proteins and denatures and unfolds them. While scattering studies using X-rays and neutrons have shown that this denaturation generally leads to protein-decorated SDS micelles, a different SDS-decorated polypeptide model has recently been suggested for complexes between SDS and Ubiquitin (UBI), in which individual SDS molecules are distributed on a partially stretched protein. To resolve this apparent discrepancy, we have investigated the SDS-UBI system by a number of complementary techniques. Small-angle X-ray scattering (SAXS) provides the overall structure of the SDS-UBI complexes, Tyr fluorescence and circular dichroism follow changes in tertiary and secondary structure, and isothermal titration calorimetry determines the stoichiometries of complexes and the amount of free SDS as a function of [SDS]. At low [SDS], UBI preserves its folded structure but dimerizes to a small extent. At 4 SDS per UBI, a complex is formed with two UBI and a small shared SDS cluster with 8 SDS molecules. In these complexes UBI preserves most of its native fold. At 10-12 SDS per UBI, which remains below the critical micelle concentration under our conditions, UBI-covered SDS micelles form with four UBIs around a core of 40 SDSs. This implies a protein-assisted micellization and an associated unfolding of UBI involving a change from mainly β-strands to mainly α-helical secondary structure. As [SDS] is increased, the complex gradually changes so that finally only one UBI covers one micelle with a similar number of SDS molecules at SDS saturation. Thus, we conclude that SDS unfolds UBI by mechanisms very similar to those observed for other globular proteins, leading to a protein-decorated SDS micelle rather than an SDS-decorated unfolded polypeptide chain.