Structural Investigations of Human A2M Identify a Hollow Native Conformation That Underlies Its Distinctive Protease-Trapping Mechanism

Human α2-macroglobulin (A2M) is the most characterized protease inhibitor in the alpha-macroglobulin (αM) superfamily, but the structure of its native conformation has not been determined. Here, we combined negative stain electron microscopy (EM), small-angle X-ray scattering (SAXS), and cross-linking–mass spectrometry (XL-MS) to investigate native A2M and its collapsed conformations that are obtained through aminolysis of its thiol ester by methylamine or cleavage of its bait region by trypsin. The combined interpretation of these data resulted in a model of the native A2M tetramer and its conformational changes. Native A2M consists of two crescent-shaped disulfide-bridged subunit dimers, which face toward each other and surround a central hollow space. In native A2M, interactions across the disulfide-bridged dimers are minimal, with a single major interface between the linker (LNK) regions of oppositely positioned subunits. Bait region cleavage induces both intrasubunit domain repositioning and an altered configuration of the disulfide-bridged dimer. These changes collapse the tetramer into a more compact conformation, which encloses an interior protease-trapping cavity. A recombinant A2M with a modified bait region was used to map the bait region’s position in native A2M by XL-MS. A second recombinant A2M introduced an intersubunit disulfide into the LNK region, demonstrating the predicted interactions between these regions in native A2M. Altogether, our native A2M model provides a structural foundation for understanding A2M’s protease-trapping mechanism, its conformation-dependent receptor interactions, and the dissociation of native A2M into dimers due to inflammatory oxidative stress.
Graphical Abstract
Highlights • Native A2M is hollow and tube-like. • A2M’s bait regions are oriented inward and are accessed from inside A2M. • A2M tetramerizes through symmetrical interactions between its LNK regions. • The receptor-binding site is in an inaccessible position inside native A2M.
In Brief The native conformation of the protease inhibitor A2M was investigated using negative stain electron microscopy, small-angle X-ray scattering, and cross-linking mass spectrometry. The low-resolution model built from these data describes a hollow tubular configuration that explains several aspects of A2M’s unique trapping mechanism. This model was further validated by two recombinantly expressed A2M mutants, which probed the location of the bait region and demonstrated the existence of a critical interface between A2M’s disulfide-bridged dimers.