Demonstration by burst-phase analysis of a robust folding intermediate in the FF domain.

The role of intermediates in the folding reaction of single-domain proteins is a controversial issue. It was previously shown by different methods that an on-pathway intermediate is populated in the presence of sodium sulphate during the folding of the FF domain from HYPA/FBP11. Here we demonstrate using analysis of the amplitudes of kinetic traces that this burst-phase folding intermediate is present at different salt concentration and at various pH, and is also found in roughly 30 site-directed mutants. The intermediate appears robust to changing conditions and thus fulfils an important criterion for a productive molecular species on the folding reaction pathway.