BINDING OF NEUROTROPHIN-3 TO P75(LNGFR), TRKA, AND TRKB MEDIATED BY ASINGLE FUNCTIONAL EPITOPE DISTINCT FROM THAT RECOGNIZED BY TRKC

Neurotrophins regulate differentiation and survival of vertebrate neur ons through binding to members of the Trk family of receptor tyrosine kinases and to a common low affinity receptor, p75(LNGFR). The specifi city of neurotrophin action is determined by their selective interacti on with the different members of the Trk family; TrkA, TrkB, and TrkC serve as cognate receptors for nerve growth factor, brain-derived neur otrophic factor, and neurotrophin-3 (NT-3), respectively, Unlike nerve growth factor and brain-derived neurotrophic factor, NT-3 can to some extent also bind and activate non-cognate TrkA and B receptors, altho ugh the physiological relevance of these interactions is unclear. Prev ious studies established that neurotrophins use an extended surface fo r binding to cognate Trk receptors, while binding to p75(LNGFR) is med iated by a localized cluster of positively charged residues, Here we s how that the binding site of NT-3 to its non-preferred receptors TrkA and TrkB is dominated by two positively charged residues, Arg-31 and H is-33, previously shown to constitute a main determinant of binding to p75(LNGFR). Simultaneous mutation of these two residues into Ala comp letely abolished NT-3 binding and signaling through TrkA and greatly d iminished binding and activation of TrkB, However, NT-3 binding and si gnaling through its cognate receptor TrkC was unaffected by the mutati on, These results show that binding of NT-3 to p75(LNGFR), TrkA, and T rkB is mediated by a common determinant, which is distinct from that r ecognized by TrkC and also different and more localized than the one r ecognized by TrkA and TrkB in their cognate ligands, Thus, although ho mologous regions in all neurotrophins are used for binding to Trk rece ptors, a given Trk may actually contact different residues in differen t neurotrophins. The mutant NT-3 described here may be of greater adva ntage than native NT-3 when a trophic activity needs to be specificall y targeted to TrkC-expressing neurons and provides a monospecific neur otrophin for future therapeutic development.