Integrated Native Mass Spectrometry Imaging of Soluble and Membrane Proteins

Native ambient mass spectrometry enables the analysis of intact protein complexes directly from fresh frozen tissue sections together with visualization of their spatial distribution as part of a mass spectrometry imaging workflow. Native mass spectrometry imaging typically employs nanospray-desorption electrospray ionization (nano-DESI), a liquid junction sampling approach. Imaging of both soluble and membrane proteins has been demonstrated by native nano-DESI but, crucially, imaging of one protein type has always been at the expense of the other, requiring tailored sample preparation and multiple tissue sections. Here, we introduce a new mode of nano-DESI operation that combines soluble and membrane protein analysis into a single experiment, requiring no sample preparation and only a single tissue section, and which is compatible with mass spectrometry imaging. Chromatography-like separation of soluble and membrane protein signals, observed as varying elution profiles, occurs when the nano-DESI probe is parked in a fixed location on the tissue. The elution profiles of proteins in both kidney and brain tissue were explored. The results show that elution profiles are quick to record, offer insight into the classification of unknown proteins detected from tissue and enable signal-to-noise improvements to imaging and native top-down mass spectrometry workflows.