2 RESTRICTED SITES ON THE SURFACE OF THE NERVE GROWTH-FACTOR MOLECULEINDEPENDENTLY DETERMINE SPECIFIC TRKA RECEPTOR-BINDING AND ACTIVATION

Nerve growth factor (NGF) and neurotrophin-3 (NT-3) mediate activities such as survival, differentiation, and proliferation in various subse ts of neurons, In this report, we define precisely the residues in hum an NGF responsible for NGF biological activity and binding specificity to the neurotrophin receptor TrkA. In earlier studies we defined five amino acid residues of NGF which confer NGF-like activity to NT-3 whe n replacing corresponding residues in the 120-amino acid long NT-3 mol ecule. Using this gain-of-function strategy we report the further diss ection of this functional epitope. We also define another motif separa ted topographically in the NGF dimer and determined to be independentl y responsible for NGF specificity. The first of the two motifs determi ned to elicit NGF specificity is defined by the residues Val-48, Pro-4 9, and Gln-96, which are situated in the two top beta-loops of NGF. Th e second motif is represented by residues Pro-5 and Phe-7 situated in the proximal part of the NH2 terminus. Both motifs contain structurall y important residues revealing a novel principle, where specificity fo r neurotrophin ligand-receptor interactions could be determined by var iable residues modifying the conformation of the neurotrophin backbone . These findings will enhance further the possibility of mimicking NGF with low molecular weight compounds.