Structural analysis of acute-phase alpha 2-macroglobulin.

High resolution images of rat acute-phase alpha 2-macroglobulin (AP alpha 2M) have been obtained by using dark-field electron microscopy. No staining or artifact-inducing procedures were used. Analysis of unfiltered electron microscope plates, exposed to minimal electron beam radiation, revealed highly contrasted particles of variable morphology with dimensions of approx. 19 nm X 14 nm. An electron-dense core with four to six projections could be seen. Two-fold symmetry was evident in selected images, supporting the four-subunit composition of the protein. Image processing and filtering confirmed the presence and configuration of the projections by demonstrating exact molecular dimensions of 16 nm X 9.5 nm and a shape with six projections like that of the Russian letter zh. SDS/polyacrylamide-gel electrophoresis revealed that this molecule was in the proteinase-bound form. C.d. data revealed a surprisingly low content of alpha-helical secondary structure (12%) and an atypically large content of beta-form structure (33%). Comparison of the amino acid compositions of AP alpha 2M and human alpha 2-macroglobulin indicated a high degree of homology between the two molecules. It is concluded that the conformation of rat AP alpha 2M, both at the molecular and secondary structural levels, is strikingly similar to that of human alpha 2-macroglobulin.