title: The conundrum of the high-affinity NGF binding site formation unveiled?
abstract: The homodimer NGF (nerve growth factor) exerts its neuronal activity upon binding to either or both distinct transmembrane receptors TrkA and p75(NTR). Functionally relevant interactions between NGF and these receptors have been proposed, on the basis of binding and signaling experiments. Namely, a ternary TrkA/NGF/p75(NTR) complex is assumed to be crucial for the formation of the so-called high-affinity NGF binding sites. However, the existence, on the cell surface, of direct extracellular interactions is still a matter of controversy. Here, supported by a small-angle x-ray scattering solution study of human NGF, we propose that it is the oligomerization state of the secreted NGF that may drive the formation of the ternary heterocomplex. Our data demonstrate the occurrence in solution of a concentration-dependent distribution of dimers and dimer of dimers. A head-to-head molecular assembly configuration of the NGF dimer of dimers has been validated. Overall, these findings prompted us to suggest a new, to our knowledge, model for the transient ternary heterocomplex, i.e., a TrkA/NGF/p75(NTR) ligand/receptors molecular assembly with a (2:4:2) stoichiometry. This model would neatly solve the problem posed by the unconventional orientation of p75(NTR) with respect to TrkA, as being found in the crystal structures of the TrkA/NGF and p75(NTR)/NGF complexes.
authors: Istituto di Cristallografia, Consiglio Nazionale delle Ricerche, Trieste, Italy.
authors: Sonia
authors: S
authors: Covaceuszach
authors: European Molecular Biology Laboratory, Hamburg Outstation, Hamburg, Germany; Institute of Crystallography, Russian Academy of Sciences, Moscow, Russia.
authors: Petr V
authors: PV
authors: Konarev
authors: Istituto di Cristallografia, Consiglio Nazionale delle Ricerche, Trieste, Italy.
authors: Alberto
authors: A
authors: Cassetta
authors: European Brain Research Institute, Roma, Italy.
authors: Francesca
authors: F
authors: Paoletti
authors: European Molecular Biology Laboratory, Hamburg Outstation, Hamburg, Germany.
authors: Dmitri I
authors: DI
authors: Svergun
authors: Istituto di Cristallografia, Consiglio Nazionale delle Ricerche, Trieste, Italy. Electronic address: doriano.lamba@ts.ic.cnr.it.
authors: Doriano
authors: D
authors: Lamba
authors: European Brain Research Institute, Roma, Italy; Scuola Normale Superiore, Pisa, Italy. Electronic address: antonino.cattaneo@sns.it.
authors: Antonino
authors: A
authors: Cattaneo
mesh_terms: D001665:Binding Sites; D006801:Humans; D006860:Hydrogen Bonding; D056004:Molecular Dynamics Simulation; D020932:Nerve Growth Factor / Q000737:chemistry / Q000378:metabolism*; D055503:Protein Multimerization; D020917:Receptor, trkA; D053838:Scattering, Small Angle; D014961:X-Ray Diffraction
chemical_list: D020932:Nerve Growth Factor; D020917:Receptor, trkA
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