Native Ambient Mass Spectrometry of an Intact Membrane Protein Assembly and Soluble Protein Assemblies Directly from Lens Tissue

Membrane proteins constitute around two‐thirds of therapeutic targets but present a significant challenge for structural analysis due to their low abundance and solubility. Existing methods for structural analysis rely on over‐expression and/or purification of the membrane protein, thus removing any links back to actual physiological environment. Here, we demonstrate mass spectrometry analysis of an intact oligomeric membrane protein directly from tissue. Aquaporin‐0 exists as a 113 kDa tetramer, with each subunit featuring six transmembrane helices. We report the characterisation of the intact assembly directly from a section of sheep eye lens without sample pre‐treatment. Protein identity was confirmed by mass measurement of the tetramer and subunits, together with top‐down mass spectrometry, and the spatial distribution was determined by mass spectrometry imaging. Our approach allows simultaneous analysis of soluble protein assemblies in the tissue. The integral membrane protein assembly Aquaporin‐0 was analysed by native ambient mass spectrometry of eye lens tissue. Membrane protein assemblies were solubilised by detergent micelles. Spatially‐resolved analysis for mapping protein assembly distributions, accurate intact mass determination, and the identification of assemblies approx. 100 kDa in molecular weight are demonstrated.