Liquid Extraction Surface Analysis for Native Mass Spectrometry: Protein Complexes and Ligand Binding

Native liquid extraction surface analysis (LESA) mass spectrometry enables the direct sampling of protein complexes from a solid surface. We have previously demonstrated native LESA mass spectrometry of holomyoglobin (∼17 kDa) from glass slides and tetrameric haemoglobin (∼64 kDa) from dried blood spots and thin tissue sections. Here, we further explore the capabilities of this emerging technique by investigating a range of proteins which exist in various oligomeric states in vivo. Tetrameric avidin (∼64 kDa), octameric (∼190 kDa) and hexadecameric (∼380 kDa) CS2 hydrolase, and tetradecameric GroEL (∼800 kDa) were all detected by native LESA mass spectrometry. Moreover, trimeric AmtB, a membrane protein, could also be observed by native LESA mass spectrometry. The suitability of LESA mass spectrometry for probing protein-ligand binding was also investigated. Non-covalent complexes of the ligand biotin with the proteins avidin, haemoglobin and bovine serum albumin were detected. The results indicate that non-specific binding is minimal and that native LESA mass spectrometry is a promising tool for the investigation of biologically significant ligand binding.