Protein-metal ion interactions, stoichiometries and relative affinities determined by on-line size exclusion gel filtration mass spectrometry

The modulation of metal ions on protein function is well recognized and of paramount importance in protein biochemistry. To date, very few methods allow direct determination of protein-metal ion interactions, as well as exact stoichiometric binding ratios. In this work we demonstrate the usefulness of two on-line size exclusion gel filtration mass spectrometry approaches to directly detect protein-metal ion adducts, as well as determine exact protein-metal ion stoichiometries. We show that on-line size exclusion column chromatography (SEC) and rapid in-line desalting (RILED) coupled to microelectrospray mass spectrometry (μESI-MS) can be used for such analyses. The SEC approach can be effectively used to both separate proteins in a complex mixture and exchange buffers prior to the electrospray process. While RILED does not allow for protein separation, it provides a much faster high-throughput desalting procedure than the conventional SEC technique. Specifically, we show that SEC/μESI-MS and RILED/MS can be used to determine calcium ion binding stoichiometries to a high-affinity, metal ion binding protein, calbindin D_28K. Furthermore, the same approaches can also be used to determine metal ion binding stoichiometries of low-affinity metal-binding proteins such as Spo0F.