Table of drosophila strains used in this study.

<div> In <i>Drosophila</i>, two interacting adhesion protein families, Defective proboscis responses (Dprs) and Dpr interacting proteins (DIPs), coordinate the assembly of neural networks. While intercellular DIP::Dpr interactions have been well characterized, DIPs and Dprs are often co-expressed within the same cells, raising the question as to whether they also interact in <i>cis</i>. We show, in cultured cells and in vivo, that DIP-α and DIP-δ can interact in <i>cis</i> with their ligands, Dpr6/10 and Dpr12, respectively. When co-expressed in <i>cis</i> with their cognate partners, these Dprs regulate the extent of <i>trans</i> binding, presumably through competitive <i>cis</i> interactions. We demonstrate the neurodevelopmental effects of <i>cis</i> inhibition in fly motor neurons and in the mushroom body. We further show that a long disordered region of DIP-α at the C-terminus is required for <i>cis</i> but not <i>trans</i> interactions, likely because it alleviates geometric constraints on <i>cis</i> binding. Thus, the balance between <i>cis</i> and <i>trans</i> interactions plays a role in controlling neural development. </div>