Characterization of the solution structure of a neuroligin/β-neurexin complex

Neuroligins are post-synaptic cell adhesion molecules that promote synaptic maturation and stabilization upon binding with pre-synaptic partners, the α- and β-neurexins. Using a combination of analytical ultracentrifugation, small angle X-ray, and neutron scattering, we have characterized the low-resolution three-dimensional structure of the extracellular domain of the neuroligins, free in solution, and in complex with β-neurexin. The globular extracellular domain of the neuroligins forms stable homodimers through a four-helix bundle typical of the cholinesterases and other members of the α/β-hydrolase fold family. The presence of the stalk region adds to the extracellular domain of neuroligin-1 an elongated structure, suggesting a rod-like nature of the stalk domain. Sedimentation equilibrium coupled with solution scattering data of the β-neurexin/neuroligin-1 complex indicated a 2:2 stoichiometry where two β-neurexin molecules bind to a neuroligin-1 dimer. Deuteration of neurexin allowed us to collect neutron scattering data that, in combination with other biochemical techniques, provide a basis for optimizing the positioning of each component in a detailed computational model of the neuroligin/neurexin complex. As several mutations of both neurexin and neuroligin genes have been linked to autism spectrum disorders and mental retardation, these new structures provide an important framework for the study of altered structure and function of these synaptic proteins.