National Center for Protein Science Shanghai, State Key Laboratory of Molecular Biology, Center for Excellence in Molecular Cell Science, Institute of Biochemistry and Cell Biology, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 320 Yueyang Road, Shanghai 200031, P.R. China.
National Center for Protein Science Shanghai, State Key Laboratory of Molecular Biology, Center for Excellence in Molecular Cell Science, Institute of Biochemistry and Cell Biology, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 320 Yueyang Road, Shanghai 200031, P.R. China.
Cell Rep. 2017 Sep 5;20(10):2328-2340. doi: 10.1016/j.celrep.2017.08.031.
Unlike cerebellin 1 (Cbln1), which bridges neurexin (Nrxn) receptors and δ-type glutamate receptors in a trans-synaptic triad, Cbln4 was reported to have no or weak binding for the receptors despite sharing ∼70% sequence identity with Cbln1. Here, we report crystal structures of the homotrimers of the C1q domain of Cbln1 and Cbln4 at 2.2 and 2.3 Å resolution, respectively. Comparison of the structures suggests that the difference between Cbln1 and Cbln4 in GluD2 binding might be because of their sequence and structural divergence in loop CD. Surprisingly, we show that Cbln4 binds to Nrxn1β and forms a stable complex with the laminin, nectin, sex-hormone binding globulin (LNS) domain of Nrxn1β. Furthermore, the negative-stain electron microscopy reconstruction of hexameric full-length Cbln1 at 13 Å resolution and that of the Cbln4/Nrxn1β complex at 19 Å resolution suggest that Nrxn1β binds to the N-terminal region of Cbln4, probably through strand β10 of the S4 insert.
与在突触间三联体中连接神经连接蛋白（Nrxn）受体和 δ 型谷氨酸受体的小脑蛋白 1（Cbln1）不同，尽管 Cbln4 与 Cbln1 具有约 70%的序列同一性，但它与受体的结合能力很弱或没有。在这里，我们分别以 2.2 和 2.3Å 的分辨率报告了 Cbln1 和 Cbln4 的 C1q 结构域同源三聚体的晶体结构。结构比较表明，Cbln1 和 Cbln4 在 GluD2 结合方面的差异可能是由于它们在环 CD 中的序列和结构差异。令人惊讶的是，我们表明 Cbln4 与 Nrxn1β 结合并形成与 Nrxn1β 的层粘连蛋白、神经节苷脂、性激素结合球蛋白（LNS）结构域的稳定复合物。此外，在 13Å 分辨率下对六聚体全长 Cbln1 的负染电子显微镜重构和在 19Å 分辨率下对 Cbln4/Nrxn1β 复合物的负染电子显微镜重构表明，Nrxn1β 可能通过 S4 插入的β10 链结合到 Cbln4 的 N 端区域。
