title: Structural basis for variant-specific neuroligin-binding by α-neurexin.
abstract: 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. α-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 β-Nrx has but a single LNS domain. It has long been known that the larger α-Nrx and the shorter β-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 Nrx1α ectodomain, consisting of LNS5-EGF3-LNS6. The 2.3 Å-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 α-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 α-Nrx. This, combined with a low-resolution molecular envelope obtained by a single particle reconstruction performed on negatively stained full-length Nrx1α sample, allowed us to derive a structural model of the α-Nrx ectodomain. This model will help us understand not only how the large α-Nrx ectodomain is accommodated in the synaptic cleft, but also how the trans-synaptic adhesion mediated by α- and β-Nrxs could differentially affect synaptic structure and function.
authors.affiliation: Laboratory of Protein Synthesis and Expression, Institute for Protein Research, Osaka University, Suita, Osaka, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Tanaka
authors.forename: Terukazu
authors.initials: T
authors.lastname: Nogi
authors.forename: Norihisa
authors.initials: N
authors.lastname: Yasui
authors.forename: Kenji
authors.initials: K
authors.lastname: Iwasaki
authors.forename: Junichi
authors.initials: J
authors.lastname: Takagi
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D015816:Cell Adhesion Molecules, Neuronal / Q000378:metabolism*; D005109:Extracellular Matrix / Q000378:metabolism; D006023:Glycoproteins / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009479:Neuropeptides / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding; D020033:Protein Isoforms / Q000378:metabolism; D017434:Protein Structure, Tertiary; D013379:Substrate Specificity; D013569:Synapses / Q000378:metabolism
chemical_list: D015816:Cell Adhesion Molecules, Neuronal; D006023:Glycoproteins; D009479:Neuropeptides; D020033:Protein Isoforms; C112162:neurexophilin; C093438:neuroligin 1