Preparation and characterization of a C-terminal fragment of pregnancy zone protein corresponding to the receptor-binding peptide from human α2-macroglobulin

Abstract

Digestion of the pregnancy zone protein with papain at pH 4.5 yields an 18 kDa C-terminal fragment. This fragment consists of the 145 C-terminal amino-acid residues cleaved at Asn- 1288Ile and is homologous to the C-terminal receptor binding fragment of human α 2-macroglobulin obtained by cleavage with papain. The fragment contains an intrachain disulfide bond between 1308Cys and 1423Cys corresponding to that between 1304Cys and 1419Cys in α 2-macroglobulin. An oligosaccharide chain, is present in the C-terminal fragment of pregnancy zone protein as in human α 2-macroglobulin. The PZP C-terminal fragment was demonstrated to bind to the LRP/ α 2M-receptor. Both the pregnancy zone protein and α 2-macroglobulin fragments bind three mAb's ( α1:1, R35, and 7H11D6) generated against α 2-macroglobulin. The mAb 7H11D6 was generated against the α 2-macroglobulin-proteinase complex (Isaacs, I.J., Steiner, J.P., Roche, P.A., Pizzo, S.V. and Strickland, D.K. (1988) J. Biol. Chem. 263, 6709–6714) and the binding of this to the C-terminal fragments of both pregnancy zone protein and α 2-macroglobulin indicates that both proteins use the same receptor recognition site for binding to the LRP/ α 2M-receptor.