Structural characterization of a highly-ordered 'molten globule' at low pH.

The characterization of unfolded and partly folded states of proteins is central to understanding protein stability and folding, as well as providing a basis for protein design. The four helix bundle-protein interleukin-4 undergoes an unfolding transition at low pH. Using heteronuclear nuclear magnetic resonance methods we show that following this transition the protein retains a highly ordered hydrophobic core in which most, but not all, of the secondary structure is preserved. Extensive disorder exists, however, in regions of polypeptide chain linking the structural elements which make up this core. We suggest that this 'highly ordered molten globule' could be indicative of the type of structures occurring late in protein folding processes, in contrast to more disordered 'molten globules' which relate to early folding intermediates.