Crosstalk between Echinoid and Sidekick, two IgCAM proteins, modulates Adherens Junction dynamics and tissue remodelling

Adherens junctions (AJs) undergo dynamic remodelling during epithelial morphogenesis, requiring precise coordination between adhesive proteins, intracellular adaptors, and cytoskeletal regulators. In addition to cadherins, which mediate core cell-cell adhesion and connect junctions to the actin cytoskeleton, other adhesion molecules from the immunoglobulin superfamily (IgCAM) also contribute to AJ organisation. In Drosophila, Echinoid (Ed), a nectin-like IgCAM, localises along the entire AJ, whereas Sidekick (Sdk), another IgCAM, is predominantly enriched at tricellular adherens junctions (tAJs). Although both proteins interact with overlapping intracellular partners, how they functionally relate to one another has remained unclear. Here, we investigate the spatial, molecular, and functional interplay between Sdk and Ed during embryonic epithelial morphogenesis. Using SRRF-imaging we show that Sdk and Ed frequently colocalise at tAJs but also display adjacent or spatially separated distributions; together with proximity-labelling experiments, these results suggest that Sdk and Ed engage in transient and dynamic associations rather than forming a stable complex. Functional analyses reveal that they influence each others accumulation, indicating bidirectional regulatory interactions. We find that Sdk modulates Ed levels along the entire AJs and affects Ed enrichment at tAJs. We provide evidence that this regulation involves changes in Ed intracellular trafficking, suggesting that Sdk modulates Ed levels at AJs at least in part by controlling its trafficking. Genetic analyses uncover previously unreported contributions of ed to tracheal cell intercalation and of sdk to dorsal closure, and reveal strong genetic interactions between the two genes, indicating cooperative yet context-dependent functions. Consistent with this, we find that Sdk and Ed converge on shared intracellular adaptor proteins, including Canoe and Polychaetoid, modulating their levels and junctional enrichment. Together, our findings support a model in which dynamic, multi-component protein complexes assemble at bicellular and tricellular AJs, integrating shared and junction-enriched components that engage in multiple, simultaneous, and mutually influencing interactions. This interconnected network would confer the spatiotemporal robustness and flexibility required to support the distinct cellular behaviours underlying tissue-specific remodelling.