Structural characterization of the extracellular domain of alpha‐neurexin:insights into synapse assembly

Neurexins are a family of pre‐synaptic cell‐adhesion membrane proteins that are important for synapse maintenance through contact with the postsynaptic protein, neuroligin. Several studies indicate that mutations in these genes may play an important role in autism and mental retardation. Each neurexin gene contains two promoters generating α‐ and β‐neurexins. Whereas the extracellular domain of β‐neurexin contains only a single LNS domain, α‐neurexin consists of six LNS domains, including the β‐neurexin domain, separated by three EGF domains. We have used single particle EM and small angle x‐ray scattering to obtain low‐resolution images of the extracellular domain of α‐neurexin, which offer insights into the macromolecular structure. Furthermore, epitope tagging by antibodies reveal the N‐ and C‐terminus. Using rigid body modeling and ab‐initio restoration approach, we have determined a structural model of the entire extracellular domain of α‐neurexin. The crystal structures of the individual LNS domains in neurexin and the β‐neurexin‐neuroligin complex supplement the analysis. Together with analytical ultracentrifugation, mass spectrometry, and gel filtration, we now have an understanding of the basic biophysical properties of α‐neurexin. These results show that α‐neurexin exists as a glycosylated monomer, with an extended conformation and some inherent flexibility between the LNS domains.