Top-Down Ion Mobility Mass Spectrometry Reveals a Disease Associated Conformational Ensemble of Alpha-1-antitrypsin

Mutants of members of the serpin superfamily can undergo nonamyloid aggregation to form polymeric chains that are associated with disease. This is typified by Z alpha-1-antitrypsin (Glu342Lys) that accumulates as polymers within hepatocytes to cause cirrhosis. We have used ion mobility mass spectrometry and electron-capture dissociation to directly observe and characterize novel intermediates formed during polymerization. Our data are congruent with an ensemble of conformations that are monomeric but maintained in a partially misfolded metastable state in which ∼12% of the molecule at the C-terminus is displaced. The application of these techniques to Z alpha-1-antitrypsin polymers isolated from human liver revealed a molecular species most consistent with a polymer mediated by an intermolecular C-terminal domain insertion. These findings establish a previously unobserved progression of pathogenic structural changes and thereby extend the mechanism of alpha-1-antitrypsin polymerization. They additionally demonstrate the strengths of native top-down ion mobility mass spectrometry in characterizing misfolding intermediates and proteins isolated from human tissue.