Structural basis for the self-recognition of sDSCAM in Chelicerata.

To create a functional neural circuit, neurons develop a molecular identity to discriminate self from non-self. The invertebrate Dscam family and vertebrate Pcdh family are implicated in determining synaptic specificity. Recently identified in Chelicerata, a shortened Dscam (sDscam) has been shown to resemble the isoform-generating characters of both Dscam and Pcdh and represent an evolutionary transition. Here we presented the molecular details of sDscam self-recognition via both trans and cis interactions using X-ray crystallographic data and functional assays. Based on our results, we proposed a molecular zipper model for the assemblies of sDscam to mediate cell-cell recognition. In this model, sDscam utilized FNIII domain to form side-by-side interactions with neighboring molecules in the same cell while established hand-in-hand interactions via Ig1 domain with molecules from another cell around. Together, our study provided a framework for understanding the assembly, recognition, and evolution of sDscam.