Conformational variants of human alpha2-macroglobulin are reflected in a C-terminal 'switch region'

Human alpha2-macroglobulin displays extensive conformational changes when induced to transform into new quaternary structures, which are eliminated from the systemic circulation by receptor-mediated endocytosis. One major region involved in these conformational changes is located in a segment of 30 amino acids from Glu1314 to Ser1343 (-Glu-Glu-Phe-Pro-Phe-Ala-Leu-Gly-Val-Gln-Thr-Leu-Pro-Gln-Thr-Cys-Asp -Glu-Pro-Lys-Ala-His-Thr-Ser-Phe-Gln-Ile-Ser-Leu-Ser-), which we term the 'switch region' of alpha2-macroglobulin, as deduced by immunochemical techniques. Monoclonal antibodies were generated using either native, methylamine-treated or the 18-kDa C-terminal receptor-binding fragment as the immunogen. From an extensive number of obtained hybridomas, 11 mAbs were selected because of their capacity to bind to the C-terminal fragment. Irrespective of the original configuration of the antigen used for immunization, seven of the antibodies were shown to be reactive with a set of overlapping epitopes, closely positioned within the 'switch region', as confirmed by the use of synthetic peptides covering the entire C-terminal fragment. The specificities of the seven individual antibodies, as determined by ELISA and BIAcore technologies, revealed a pronounced conformational pleomorphism in the 'switch region'. The results indicate that the 'switch region' may be involved in the exposure of the receptor recognition site and can be used as an indicator region for different conformational states of alpha2-macroglobulin.