An ICP-MS, ESI-MS and molecular modelling investigation of homogeneous gallium affinity tagging (HMAT) of phosphopeptides

Protein phosphorylation and de-phosphorylation, provide one of the most common signalling pathways within cells, being involved in regulating cellular processes, mediating enzyme inhibition, protein-protein recognition and protein degradation. Compared with normal proteomics, phosphoproteomics poses some additional challenges requiring more initial separation and additional sensitivity to detect and quantify potentially ultra-low abundance species. In this work, the selective detection of phosphopeptides is described based on the incorporation of a metal tag, gallium-N,N-biscarboxymethyl lysine (Ga-LysNTA), in solution before separation and detection by liquid chromatography coupled to inductively coupled plasma mass spectrometry (LC-ICP-MS). Experimental and theoretical characterisation of the resulting Ga-phosphopeptide complex is presented based on linear ion trap electrospray ionisation mass spectrometry (ESI-MS), Fourier transform mass spectrometry (FT-MS) and molecular modelling data. Linear ion trap electrospray ionisation mass spectrometry (ESI-MS) was employed to study the interaction of the gallium tag with platelet derived growth factor beta receptor (β-PDGF), a small phosphopeptide. In addition high resolution Fourier transform mass spectrometry (FT-MS) was used for accurate mass determination and multistage tandem mass spectrometry of the gallium-β-PDGF complex identified the fragmentation pathway. Finally, molecular modelling was used to investigate the energetically favoured structures of both the Ga-LysNTA material and the β-PDGF-Ga-LysNTA complex.