Electron microscope studies of human alpha 2-macroglobulin-chymotrypsin complex: demonstration that the two structures assigned to native and proteolyzed alpha 2-macroglobulin represent two views of the proteolyzed molecule

Electron microscope studies of native and protease-bound human alpha 2-macroglobulin have led to two contradictory models for these two structures. One viewpoint maintains that the native structure has the shape of )+(, which contracts on binding of the protease to the shape of (). An opposing view proposes that the native structure has the shape of a padlock and that )+( and () are the side and end views of the proteolyzed molecule. In this investigation, electron microscope studies of the alpha-chymotrypsin-treated alpha 2-macroglobulin utilizing a tilt stage have shown that the two shapes )+( and ()] interconvert. This demonstrates that these two shapes represent the side and end views of the proteolyzed alpha 2-macroglobulin which are related by a 90 degree rotation of the prototype molecule.