Crystal structure of the neurotrophin-3 and p75~(NTR) symmetrical complex

摘要

Neurotrophins (NTs) are important regulators for the survival, differentiation and maintenance of different peripheral and central neurons. NTs bind to two distinct classes of glycosylated receptor: the p75 neurotrophin receptor (p75~(NTR)) and tyrosine kinase receptors (Trks). Whereas p75~(NTR) binds to all NTs, the Trk subtypes are specific for each NT. The question of whether NTs stimulate p75~(NTR) by inducing receptor homodimerization is still under debate. Here we report the 2.6-A resolution crystal structure of neurotrophin-3 (NT-3) complexed to the ectodomain of glycosylated p75~(NTR). In contrast to the previously reported asymmetric complex structure, which contains a dimer of nerve growth factor (NGF) bound to a single ectodomain of deglycosylated p75~(NTR) (ref. 3), we show that NT-3 forms a central homodimer around which two glycosylated p75~(NTR) molecules bind symmetrically. Symmetrical binding occurs along the NT-3 interfaces, resulting in a 2:2 ligand-receptor cluster. A comparison of the symmetrical and asymmetric structures reveals significant differences in ligand-receptor interactions and p75~(NTR) conformations. Biochemical experiments indicate that both NT-3 and NGF bind to p75~(NTR) with 2:2 stoichiometry in solution, whereas the 2:1 complexes are the result of artificial deglycosylation. We therefore propose that the symmetrical 2:2 complex reflects a native state of p75~(NTR) activation at the cell surface. These results provide a model for NTs-p75~(NTR) recognition and signal generation, as well as insights into coordination between p75~(NTR) and Trks.