The transforming growth factors-beta constitute a family ofmulti-functional cytokines that regulate cell growth and differentiation []. Many cells synthesise TGF-beta, and essentially all have specific receptors for this peptide []. TGF-beta regulates the actions of many other peptide growth factors and determines a positive or negative direction of their effects. The protein functions as a disulphide-linked homodimer. Its sequence is characterised by the presence of several C-terminal cysteine residues, which form interlocking disulphide links arranged in a knot-like topology. A similar "cystine-knot"arrangement has been noted in the structures of some enzyme inhibitors and neurotoxins that bind to voltage-gated Ca2+ channels, although the precise topology differs.The three-dimensional structures of several members of the TGF-beta super-family have been deduced [, , ]. TGF-beta genes are expressed differentially, suggesting that the various TGF-beta species may have distinct physiological roles in vivo.The complete amino acid sequence of human beta 2 transforming growth factor(hTGF-beta 2) has been determined by automated Edman degradation []. Human TGF-beta 2 consists of 2 identical disulphide-linked subunits that share a high degree of similarity with the functionally related TGF-beta 1, and reveal lower levels of similarity to porcine inhibins and activins, the C-terminal regions of human Mullerian inhibiting substance, and the putativedecapentaplegic gene complex protein of Drosophila melanogaster.The crystal structure of the TGF-beta 2 monomer lacks a well-defined hydrophobic core and displays an unusual elongated non-globular fold []. Eight cysteine residues form 4 intra-chain disulphide bonds, creating the characteristic knotted arrangement. The dimer is stabilised by a ninth cysteine, which forms an inter-chain disulphide bond,and by 2 identical hydrophobic interfaces. Other members of the TGF-betasuperfamily, including activins, inhibins and various developmental factors,are also likely to adopt the TGF-beta fold.
