Heparin/heparan sulphate binding in the TGF-β cytokine superfamily

The TGF-β (transforming growth factor-β) cytokine superfamily in mammals contains some 30 members. These dimeric proteins are characterized by a strongly conserved cystine knot-based structure. They regulate the proliferation, differentiation and migration of many cell types, and therefore have important roles in morphogenesis, organogenesis, tissue maintenance and wound healing. Thus far, around onequarter of these cytokines have been shown to bind to heparin and heparan sulphate. Well-established examples are the TGF-β isoforms 1 and 2, and the BMPs (bone morphogenetic proteins) -2 and -4. In studies in my laboratory, we have shown that GDNF (glial-cell-line-derived neurotrophic factor) and its close relatives neurturin and artemin bind to heparin and heparan sulphate with high affinity. We have reported previously that binding of GDNF is highly dependent on the presence of 2-O-sulphate groups. More recently, we and others have been investigating the heparin/heparan sulphate-binding properties of BMP-7, which is a representative of a distinct BMP subgroup from that of BMPs -2 and -4. Interestingly, several of the various specific BMP antagonist proteins also bind to heparin and heparan sulphate. Much remains to be learnt about the nature and role of glycosaminoglycan interactions in the TGF-β superfamily, but current work suggests that these cytokines do not share a single highly conserved heparin/heparan sulphate-binding site. The TGF-β (transforming growth factor β) cytokine superfamily The TGF-β-related cytokines constitute a major cytokine superfamily, the first members of which appeared with the evolution of metazoans. With the emergence of higher vertebrates, there was a dramatic radiating divergent evolution of superfamily members, leading to some 30 proteins in the mammals. Despite this divergence, there has been a strong conservation of structure. The primary sequences contain seven cysteine residues with a characteristic spacing pattern. Six of these cysteines form intrachain disulphide bridges so as to give rise to an eight-residue-membered cystine ring knot [1]. The remaining cysteine forms an interchain covalent bridge which links the two monomers together in the dimeric cytokine molecule. The cystine knot structure provides the basis for a conserved polypeptide fold. The high-resolution structures of nine of the superfamily members show a tertiary structure comprising two β-strand loops stacked one above the other and extending away from one side of a short α-helix [2]. The exceptions within these structures are activin A, which lacks an α-helix [3], and TGF-β3, which has an extra-short α-helix [4]. The TGF-β cytokines are synthesized as the C-termini of large pre-proteins from which they are released by proteolytic cleavage.