Receptor‐binding domain of human α2‐macroglobulin Expression, folding and biochemical characterization of a high‐affinity recombinant derivative

Abstract
A recombinant version of the receptor binding domain (RBDv) of human α2‐macroglobulin (α2M) has been expressed in E. coli and refolded using a novel iterative procedure. RBDv (Val1299‐Ala1451) is extended by 15 residues at the N‐terminal side of the Lys1313‐Glu papain cleavage site in human α2M. RBDv contains the intra‐chain bridge Cys1329‐Cys1444 and is soluble and monomeric. Competition experiments with 125I‐labelled methylamine‐treated α2M reveal that RBDv binds to the placental receptor for transformed α2M with a K d of 8 nM, i.e. the binding affinity of RBDv is of the same order of magnitude as the intrinsic affinity for binding of one domain in transformed α2M to one receptor molecule.