Tanaka_2011_PLoS.One_6_e19411

Neurexins (Nrxs) are presynaptic membrane proteins with a single membrane-spanning domain that mediate asymmetric trans-synaptic cell adhesion by binding to their postsynaptic receptor neuroligins. alpha-Nrx has a large extracellular region comprised of multiple copies of laminin, neurexin, sex-hormone-binding globulin (LNS) domains and epidermal growth factor (EGF) modules, while that of beta-Nrx has but a single LNS domain. It has long been known that the larger alpha-Nrx and the shorter beta-Nrx show distinct binding behaviors toward different isoforms/variants of neuroligins, although the underlying mechanism has yet to be elucidated. Here, we describe the crystal structure of a fragment corresponding to the C-terminal one-third of the Nrx1alpha ectodomain, consisting of LNS5-EGF3-LNS6. The 2.3 A-resolution structure revealed the presence of a domain configuration that was rigidified by inter-domain contacts, as opposed to the more common flexible "beads-on-a-string" arrangement. Although the neuroligin-binding site on the LNS6 domain was completely exposed, the location of the alpha-Nrx specific LNS5-EGF3 segment proved incompatible with the loop segment inserted in the B+ neuroligin variant, which explains the variant-specific neuroligin recognition capability observed in alpha-Nrx. This, combined with a low-resolution molecular envelope obtained by a single particle reconstruction performed on negatively stained full-length Nrx1alpha sample, allowed us to derive a structural model of the alpha-Nrx ectodomain. This model will help us understand not only how the large alpha-Nrx ectodomain is accommodated in the synaptic cleft, but also how the trans-synaptic adhesion mediated by alpha- and beta-Nrxs could differentially affect synaptic structure and function.