Structure of the complex of C1q-like 3 protein with adhesion-GPCR BAI3.

The adhesion-GPCR Brain-specific Angiogenesis Inhibitor-3 (BAI3) plays a crucial role in organizing synapses in the brain. However, how BAI3 engages one of its ligands, the C1q-like proteins (C1qls), remains largely unexplored. Here, we present the single-particle cryo-electron microscopy (cryo-EM) structure of the C1ql3-BAI3 complex at 2.8 à resolution. The structure reveals a hexameric configuration, where C1ql3 forms a central homotrimer that effectively captures three BAI3 molecules. These BAI3 molecules fit snugly into the grooves between the trimeric C1q domains of the C1qls, employing calcium ion (Ca(2+))-mediated interactions that differ from previously characterized structures of C1q-like domain-mediated complexes. Furthermore, we conducted mutant analysis and cell surface staining, which confirmed the essential contact residues involved in this interaction. This unique binding mechanism not only enhances our understanding of the C1ql-BAI3-mediated synaptic organization but also sheds light on the functional dynamics of BAI3 in the brain.