Structural Studies of C1QL-Mediated Complexes

Abstract

Proper brain function is based on neuronal networks, which are based on synapses, the fundamental unit of neuronal communication. Synaptic adhesion proteins bind across the synaptic cleft to form complexes tethered to both pre- and post-synaptic membranes. Dysfunction of synaptic adhesion proteins cause brain disorders, ‘synaptopathies’. Members of the family of complement component 1, q subcomponent-like proteins (C1QL1-4) promote synapse organization (1, 2). The underlying mechanism is unknown. Genetic analysis on C1ql3 revealed its expression in the limbic system with phenotypes of hyperactivity, sleeping disturbances, and a deficit in forming emotional memories (3)(4). C1QLs are secreted into the synaptic cleft and bind to a post-synaptically localized G protein-coupled receptor (GPCR) called adhesion GPCR B3 (ADGRB3) (1, 2). This raises the question whether C1QL has potential to bidirectionally coordinate a trans-synaptic complex to mediate a ternary complex that influences synapse homeostasis. Moreover, C1QLs’ electrostatic surface is distinct from each other and they can form higher oligomer species (5), which together with calcium specificity dictate the nature of binding and stoichiometry, resulting in a novel mechanism of how trans-synaptic adhesion is achieved. We have conducted an unbiased screen for novel C1QL3 binding partners and have identified multiple candidates that have potential to be in vivo binding partners of C1QL3. Here we present structural characterization of C1QL3 alone and together with its complex proteins using electron microscopy and X-ray crystallography.