Native mass spectrometry imaging of intact proteins and protein complexes in thin tissue sections

Here, we present native liquid extraction surface analysis (LESA) mass spectrometry imaging of proteins and protein complexes from mouse brain and liver tissue. Intact proteins were detected in characteristically low charge states, indicating that the proteins remain folded. In brain, abundant proteins such as ubiquitin and β thymosin 4 were detected homogeneously across the tissue whereas other proteins, such as neurogranin, were localised in specific anatomical regions. In liver, we demonstrate imaging of a protein complex (tetrameric hemoglobin), as well as fatty acid binding protein. Interestingly, the use of native-like solvents enables extraction of proteins which have not previously been observed in LESA experiments employing denaturing solvents, i.e., native LESA can be applied to extend the range of proteins observed. We also present native LESA ion mobility spectrometry and show that the collision cross sections of proteins extracted from tissue may be determined by travelling wave ion mobility spectrometry. The collision cross section of the 5+ ion of ubiquitin was calculated as 1047 Å2, in good agreement with measurements of ubiquitin protein standard solutions. Collision cross sections for the 4+ ions of β-thymosin 4, β-thymosin 10 and two unidentified proteins were also calculated, together with that of a 10+ ion of an unidentified protein of molecular weight 15660 Da.