THE BINDING EPITOPES OF NEUROTROPHIN-3 TO ITS RECEPTORS TRKC AND GP75AND THE DESIGN OF A MULTIFUNCTIONAL HUMAN NEUROTROPHIN

Survival and maintenance of vertebrate neurons are influenced by neuro trophic factors which mediate their signal by binding to specific cell surface receptors. We determined the binding sites of human neurotrop hin-3 (NT-3) to its receptors trkC and gp75 by mutational analysis and compared them to the analogous interactions of nerve growth factor (N GF) with trkA and gp75. The trkC binding site extends around the centr al P-strand bundle and in contrast to NGF does not make use of non-con served loops and the six N-terminal residues. The gp75 epitope is domi nated by loop residues and the C-terminus of NT-3. A novel rapid biolo gical screening procedure allowed the identification of NT-3 mutants t hat are able to signal efficiently through the non-preferred receptors trkA and trkB, which are specific for NGF and BDNF respectively. Muta tion of only seven residues in NT-3 resulted in a human neurotrophin v ariant which bound to all receptors of the trk family with high affini ty and efficiently supported the survival of NGF-, BDNF- and NT-3-depe ndent neurons. Our results suggest that the specificity among neurotro phic factors is not solely encoded in sequence diversity, but rather i n the way each neurotrophin interacts with its preferred receptor.