Goodman, Kerry MYamagata, MasahitoJin, XiangshuMannepalli, SeethaKatsamba, Phinikoula SAhlsén, GöranSergeeva, Alina PHonig, BarrySanes, JoshuaShapiro, Lawrence2016-11-182016Goodman, Kerry M, Masahito Yamagata, Xiangshu Jin, Seetha Mannepalli, Phinikoula S Katsamba, Göran Ahlsén, Alina P Sergeeva, Barry Honig, Joshua R Sanes, and Lawrence Shapiro. 2016. “Molecular basis of sidekick-mediated cell-cell adhesion and specificity.” eLife 5 (1): e19058. doi:10.7554/eLife.19058. http://dx.doi.org/10.7554/eLife.19058.2050-084Xhttp://nrs.harvard.edu/urn-3:HUL.InstRepos:29408386Sidekick (Sdk) 1 and 2 are related immunoglobulin superfamily cell adhesion proteins required for appropriate synaptic connections between specific subtypes of retinal neurons. Sdks mediate cell-cell adhesion with homophilic specificity that underlies their neuronal targeting function. Here we report crystal structures of Sdk1 and Sdk2 ectodomain regions, revealing similar homodimers mediated by the four N-terminal immunoglobulin domains (Ig1–4), arranged in a horseshoe conformation. These Ig1–4 horseshoes interact in a novel back-to-back orientation in both homodimers through Ig1:Ig2, Ig1:Ig1 and Ig3:Ig4 interactions. Structure-guided mutagenesis results show that this canonical dimer is required for both Sdk-mediated cell aggregation (via trans interactions) and Sdk clustering in isolated cells (via cis interactions). Sdk1/Sdk2 recognition specificity is encoded across Ig1–4, with Ig1–2 conferring the majority of binding affinity and differential specificity. We suggest that competition between cis and trans interactions provides a novel mechanism to sharpen the specificity of cell-cell interactions. DOI: http://dx.doi.org/10.7554/eLife.19058.001en-UScell adhesioncrystal structurehomophilic specificityneural patterningsynaptic targetingMouseJournal Article2016-11-1810.7554/eLife.19058