INTERACTIONS OF NEUROTROPHIN-3 (NT-3), BRAIN-DERIVED NEUROTROPHIC FACTOR (BDNF), AND THE NT-3 BDNF HETERODIMER WITH THE EXTRACELLULAR DOMAINS OF THE TRKB AND TRKC RECEPTORS

Interactions of three neurotrophin dimers, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and a NT-3-BDNF heterodimer with extracellular, soluble TrkB and TrkC receptors were studied using nat ive gels, light scattering, and sedimentation equilibrium. These three neurotrophins showed binding of two TrkB receptors per neurotrophin d imer, with a tendency to dissociate into one TrkB per dimer for NT-3 a nd the heterodimer, as determined by native gels, Light scattering, an d sedimentation equilibrium. For TrkC, native gels suggested binding o f NT-3, heterodimer, and BDNF but not of nerve growth factor. Sediment ation equilibrium revealed that all three neurotrophin molecules bind to TrkC at two receptors per dimer but that BDNF binds much more weakl y and that the heterodimer has an intermediate binding strength. Light scattering/size exclusion chromatography showed complexes with two Tr kC receptors per NT-3 dimer and one TrkC per heterodimer but did not d etect binding of BDNF to TrkC. This latter result is not inconsistent with the sedimentation data, because the weak binding of BDNF to TrkC may be easily dissociated by nonspecific interactions of BDNF with the size exclusion column. The relative binding constants for these neuro trophins and the soluble receptor extracellular domains, as determined by sedimentation equilibrium, are correlated with their biological ac tivity. However, the magnitude of these binding constants is insuffici ent by similar to 3 orders of magnitude to promote receptor dimerizati on at physiologically active concentrations.