Molecular structure and dimeric organization of the Notch extracellular domain as revealed by electron microscopy.
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BACKGROUND: The Notch receptor links cell fate decisions of one cell to that of the immediate cellular neighbor. In humans, malfunction of Notch signaling results in diseases and congenital disorders. Structural information is essential for gaining insight into the mechanism of the receptor as well as for potentially interfering with its function for therapeutic purposes. METHODOLOGY/PRINCIPAL FINDINGS: We used the Affinity Grid approach to prepare specimens of the Notch extracellular domain (NECD) of the Drosophila Notch and human Notch1 receptors suitable for analysis by electron microscopy and three-dimensional (3D) image reconstruction. The resulting 3D density maps reveal that the NECD structure is conserved across species. We show that the NECD forms a dimer and adopts different yet defined conformations, and we identify the membrane-proximal region of the receptor and its ligand-binding site. CONCLUSIONS/SIGNIFICANCE: Our results provide direct and unambiguous evidence that the NECD forms a dimer. Our studies further show that the NECD adopts at least three distinct conformations that are likely related to different functional states of the receptor. These findings open the way to now correlate mutations in the NECD with its oligomeric state and conformation.