The interaction of the transforming growth factor-betas with heparin/heparan sulfate is isoform-specific

We have undertaken a comparative study of the interaction of the three mammalian transforming growth factor-betas (TGF-beta) with heparin and heparan sulfate. TGF-beta1 and -beta2, but not -beta3, bind to heparin and the highly sulfated liver heparan sulfate. These polysaccharides potentiate the biological activity of TGF-beta1 (but not the other isoforms), whereas a low sulfated mucosal heparan sulfate fails to do so. Potentiation is due to antagonism of the binding and inactivation of TGF-beta1 by alpha2-macroglobulin, rather than by modulation of growth factor-receptor interactions. TGF-beta2.alpha2-macroglobulin complexes are more refractory to heparin/heparan sulfate, and those involving TGF-beta3 cannot be affected. Comparison of the amino acid sequences of the TGF-beta isoforms strongly implicates the basic amino acid residue at position 26 of each monomer as being a vital binding determinant. A model is proposed in which polysaccharide binding occurs at two distinct sites on the TGF-beta dimer. Interaction with heparin and liver heparan sulfate may be most effective because of the ability of the dimer to co-operatively engage two specific sulfated binding sequences, separated by a distance of approximately seven disaccharides, within the same chain.