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.