Detection of a Monomeric Intermediate Associated with Dimerization of Protein Hu by Mass Spectrometry

Protein HU belongs to the class of type II DNA-binding proteins and exists in all prokaryotes. The high sequence homology of HU among the various species suggests that the native topology in each case is very similar to the intertwined dimeric structure determined for protein HU from Bacillus stearothermophilus (HBst). In contrast to the structures, however, the stabilities of different HU variants vary considerably. Based on far-UV circular dichroism and differential scanning calorimetric data it has been reported that protein HU from Bacillus subtilis (HBsu) is unfolded in aqueous solution, but undergoes a folding transition to the native dimer on addition of salt. Here we show that, by manipulating the ionic strength, distinct conformational changes associated with the folding and dimerization of HBsu can be observed by using nanoflow electrospray ionization mass spectrometry (ESI-MS).