The MEGF8-MOSMO-MGRN1 complex employs a fishing pole architecture where a flexible solitary α-helix suspends a stabilized and precisely oriented RING domain beneath a membrane-embedded platform to ubiquitylate cell surface receptors, representing a distinct structural paradigm among membrane-spanning E3 ligases that is optimized for recognizing folded receptor substrates rather than translocating unfolded polypeptides.

Receptor-type E3 ubiquitin ligases are membrane-spanning assemblies that enable extracellular signals to directly control ubiquitylation in the cytoplasm. Despite playing widespread roles in tissue patterning and homeostasis, metabolism, and immunity, their structures and mechanisms remain poorly understood. Using cryo-electron microscopy, integrated with biophysical and functional studies, we visualized an E3 complex composed of two transmembrane proteins, MEGF8 and MOSMO, and the intracellular RING-family protein MGRN1. This MEGF8-MOSMO-MGRN1 (MMM) complex regulates left-right patterning of the body axis and the development of multiple organs, partly by attenuating signaling through the Hedgehog pathway. We find that the MMM complex functions like a fishing pole: a long, flexible helix attached to a membrane platform suspends an activated and precisely oriented RING domain—like a fishhook—to ubiquitylate the cytoplasmic surfaces of target receptors. Our structure explains how mutations in MEGF8 cause multi-organ birth defects in humans and defines a paradigm for receptor regulation by ubiquitylation.