Paradigms in the structural biology of the mitogenic ternary complex FGF:FGFR:heparin

The main achievements regarding the molecular interaction involving fibroblast growth factors (FGFs), canonical receptors (FGFRs) and the glycosaminoglycans (GAGs) heparan sulfate (HS)/heparin (Hp) are overviewed. Despite the recent works concerning the subject, conflicting paradigms in the structural biology of the resultant ternary complex FGF:FGFR:HS/Hp seem to persist up to these days. The principal dilemma, centered on the functional intermolecular complex of mitogenesis and angiogenesis, has been lasting for approximately a decade and a half since the publications of the two contradicting crystal structures, the asymmetric 2:2:1 versus the symmetric 2:2:2 complex model. When the principal results regarding this ternary complex are analyzed as a whole and through an impartial manner, conclusion heavily and reliably supports the existence and activity of both complex models. Selection of each complex is driven by multiple factors of different degrees of impact. Specificity in protein-binding motifs in ligands (although the minimal binding sequences are yet controversial), slight differences on the structure of the GAG-binding sites of FGF and of FGFR isoforms as well as on the possible ligand-induced conformational changes of FGFR are examples of these factors. Here, the structural biology of the mitogenic FGF:FGFR:HS/Hp ternary complex is revisited. Discussion is focused on the major attributes of this intermolecular complex including the existing conflicts about the righter biologically active model and information regarding ligand structure, conformation and minimal length required for binding to the growth factors and receptors. This review is very timely in light of the 100(th) anniversary of the discovery of Hp.