Epsin-Dependent Ligand Endocytosis Activates Notch by Force

Abstract

<p id="P3">DSL ligands activate Notch by inducing cleavage and shedding of the receptor ectodomain—an event that requires ligand to be endocytosed in signal-sending cells by the adaptor protein Epsin. Two classes of explanation for this unusual requirement are: (i) recycling models, in which ligand must be endocytosed to be modified or repositioned <i>before</i> it binds Notch, and (ii) pulling models, in which ligand must be endocytosed <i>after</i> it binds Notch to exert force that exposes an otherwise buried cleavage site. We demonstrate <i>in vivo</i> that ligands that cannot enter the Epsin pathway nevertheless bind Notch but fail to activate the receptor because they cannot exert sufficient force. This argues against recycling models and in favor of pulling models. Our results also suggest that once ligand binds receptor, activation depends on a competition between Epsin-mediated ligand endocytosis, which induces cleavage, and transendocytosis of ligand by receptor, which aborts the incipient signal. </p><p id="P4"> <div class="figure-container so-text-align-c"> <img alt="" class="figure" src="/document_file/3faca8a0-8db8-45f4-83d1-cf1c1f5f01af/PubMedCentral/image/nihms-917442-f0001.jpg"/> </div> </p><p id="P5">Force exerted by endocytosis induces ectodomain cleavage of Notch to initiate signaling.</p>