Structure-function relationships in the neurotrophin family.
The study of structure-function relationships in the neurotrophin family has in recent years increased our understanding of several important aspects of neurotrophin function. Site-directed mutagenesis studies have localized amino acid residues important for binding to the low-affinity (p75LNGFR), as well as to the members of the Trk family of tyrosine kinase receptors. A cluster of positively charged residues has been shown to form a surface for binding to p75LNGFR in all four neurotrophins. Differences in the spatial distribution of these charges among the different neurotrophins may explain some of their distinct binding properties. Elimination of these positive charges drastically reduces binding to p75LNGFR but not to the Trk family members, and it does not impair the biological properties of the neurotrophins in vitro, arguing that binding to and activation of Trk receptors is sufficient to mediate the biological responses of neurotrophins. In contrast, the binding sites to Trk receptors appear to be formed by discontinuous stretches of amino acid residues distributed throughout the primary sequence of the molecule. These include the N-terminus, some of the variable loop regions and a beta-strand. Despite their apparent distribution, when viewed in the three-dimensional structure of NGF, these residues appear grouped on one side of the neurotrophin dimer, delineating a continuous surface extending approximately parallel to the twofold symmetry axis of the molecule. Two symmetrical surfaces are formed along the axis of the neurotrophin dimer providing a model for ligand-mediated receptor dimerization. In the neurotrophin family, co-evolution of cognate ligands and Trk receptors has developed specific contacts through different residues in the same variable regions of the neurotrophins. Thus, binding specificity is determined by the cooperation of distinct active and inhibitory binding determinants that restrict ligand-receptor interactions. Binding determinants to the Trk receptors can be manipulated independently in a rational fashion to create neurotrophin analogues with novel ligand-binding properties. In this way, second-generation chimeric neurotrophins with multiple specificities (pan-neurotrophins) have been engineered which may have valuable applications in the treatment of neurodegeneration and nerve damage.