Identical or overlapping sequences in the primary structure of human alpha(2)-microglobulin are responsible for the binding of nerve growth factor-beta, platelet-derived growth factor-BB, and transforming growth factor-beta

alpha(2)-Macroglobulin (alpha(2)M) functions as a proteinase inhibitor and as a carrier of diverse growth factors. In this study, we localized binding sites for platelet-derived growth factor-RE (PDGF-BB) and nerve growth fac tor-beta (NGF-beta) to a linear sequence in the 180-kDa human alpha(2)M sub unit which includes amino acids 591-774. A glutathione S-transferase fusion protein containing amino acids 591-774 (FP3) bound PDGF-BB and NGF-beta in ligand blotting assays whereas five other fusion proteins, which collectiv ely include amino acids 99-590 and 775-1451 did not. The K-D values for PDG F-BB and NGF-beta binding to immobilized FP3 were 300 +/- 40 and 180 +/- 30 nM, respectively; these values were comparable with those determined using methylamine-modified alpha(2)M, suggesting that higher-order alpha(2)M str ucture is not necessary for PDGF-BB and NGF-beta binding. PDGF-BB and NGF-b eta blocked the binding of transforming growth factor-beta 1 (TGF-beta 1) t o FP3, Furthermore, murinoglobulin, which is the only known member of the a lpha-macroglobulin family that does not bind TGF-beta, also failed to bind PDGF-BB and NGF-beta. These results support the hypothesis that either a si ngle linear sequence in human alpha(2)M or overlapping sequences are respon sible for the binding of TGF-beta, PDGF-BB, and NGF-beta, even though there is minimal sequence identity between these three growth factors. FP3 block ed the binding of PDGF-BB to a purified chimeric protein, in which the extr acellular domain of the PDGF beta receptor was fused to the IgG(1) Fc domai n, and to PDGF receptors on NIH 3T3 cells. Thus, FP3 may inhibit the activi ty of PDGF-BB.