Structural studies of human alpha 2-macroglobulin: concordance between projected views obtained by negative-stain and cryoelectron microscopy.

Two views of native alpha 2-macroglobulin are revealed by electron microscopy of negatively stained samples; in one view the molecule resembles a padlock and in the other, a pair of lips. Interconversion of the two views upon tilting establishes that these are two different projected views of the same structure. Furthermore, the two views are related by a 45 degrees rotation about their major axis because they interconvert when the specimens are titled +/- 22.5 degrees. Negatively stained molecules on Butvar films present a nearly equal distribution of the two views, whereas in frozen-hydrated samples the molecules almost exclusively are oriented in the lip view. Measurements from both views indicate that the alpha 2-macroglobulin molecule is approximately 200 A long and approximately 140 A wide. Our results suggest that alpha 2-macroglobulin is composed of two protomeric units, each in the shape of a twisted letter S. These units are joined together at their ends to form a complex with point group symmetry 222. The 45 degrees interconversion angle between the lip and padlock views support this arrangement. Average images of unstained and stained lips are quite similar, indicating that the native structure is consistently preserved by the two electron microscopy procedures used in this investigation. This is substantiated by the interconversion between the lip and padlock views that occurs when the molecule is rotated 45 degrees [corrected] about its major twofold axis.