A 16-amino acid peptide from human α₂-macroglobulin binds transforming growth factor-β and platelet-derived growth factor-BB

Abstract

α₂-Macroglobulin (α₂M) is a major carrier of transforming growth factor-β (TGF-β) in vitro and in vivo. By screening glutathione S-transferase (GST) fusion proteins with overlapping sequences, we localized the TGF-β-binding site to aa 700–738 of the mature human α₂M subunit. In separate experiments, we screened overlapping synthetic peptides corresponding to aa 696–777 of α₂M and identified a single 16-mer (718–733) that binds TGF-β1. Platelet-derived growth factor-BB (PDGF-BB) bound to the same peptide, even though TGF-β and PDGF-BB share almost no sequence identity. The sequence of the growth factor-binding peptide, WDLVVVNSAGVAEVGV, included a high proportion of hydrophobic amino acids. The analogous peptide from murinoglobulin, a human α₂M homologue that does not bind growth factors, contained only three nonconservative amino acid substitutions; however, the MUG peptide failed to bind TGF-β1 and PDGF-BB. These results demonstrate that a distinct and highly-restricted site in α₂M, positioned near the C-terminal flank of the bait region, mediates growth factor binding. At least part of the growth factor-binding site is encoded by exon 18 of the α₂M gene, which is notable for a 5′ splice site polymorphism that has been implicated in Alzheimer's Disease.