Crystal structure of neurotrophin-3 homodimer shows distinct regions are used to bind its receptors

Neurotrophin-3 (NT-3) is a cystine knot growth factor that promotes the sur vival, proliferation, and differentiation of developing neurons and is a po tential therapeutic for neurodegenerative diseases. To clarify the structur al basis of receptor specificity and the role of neurotrophin dimerization in receptor activation, the structure of the NT-3 homodimer was determined using X-ray crystallography. The orthorhombic crystals diffract to 2.4 Angs trom, with dimer symmetry occurring about a crystallographic 2-fold axis. T he overall structure of NT-3 resembles that of the other neurotrophins, NGF and BDNF; each protomer forms a twisted four-stranded beta sheet, with thr ee intertwined disulfide bonds. There are notable differences, however, bet ween NT-3 and NGF in the surface loops and in three functionally important regions, shown in previous mutagenesis studies to be critical for binding. One such difference implies that NT-3's binding affinity and specificity de pend on a novel hydrogen bond between Gin 83, a residue important for bindi ng specificity with TrkC, and Arg 103, a residue crucial for binding affini ty with TrkC. NT-3's extensive dimer interface buries much of the otherwise solvent-accessible hydrophobic surface area and suggests that the dimeric state is stabilized through the formation of this hydrophobic core. A compa rison of the dimer interface between the NT-3 homodimer and the BDNF/NT-3 h eterodimer reveals similar patterns of hydrogen bonds and nonpolar contacts , which reinforces the notion that the evolutionarily conserved neurotrophi n interface resulted from the need for receptor dimerization in signal init iation.