Structure and function of RET in multiple endocrine neoplasia type 2

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   Figure 1

   RET receptor macromolecular complex assembly and structural characterization of functional domains. (A) Graphic representation of the distinct functional domains of RET: cadherin-like domains (CLD, 1–4), cysteine-rich domain (CRD), transmembrane domain (TM), juxtamembrane segment (JM), tyrosine kinase domain (TKD), C-terminal segment (tail). (B) EM structure for a RET–GDNF–GFRα ternary complex (TC) reveals a composite ligand-binding site (Kjaer et al. 2010, Goodman et al. 2014). EM maps for TC (surface, soft gray, top and lateral views) and color-coded cartoon representation of individual components of the complex are depicted. (C) Crystal structure of RET catalytic domain with the most relevant secondary structural elements and functional motifs highlighted in color code, see Fig. 2 for further details.
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   Figure 2

   Conserved structural and functional features of RET catalytic domain. Diagram of RET catalytic domain with known secondary structural element and functional motifs and prototypical interactions with ATP and a substrate (based on PKA catalytic subunit crystal structure PDB 1ATP). Color-coded secondary structural elements and key residues are highlighted. Coordinating Mn²⁺ is represented by two colored spheres and a tyrosine instead of an alanine has been modeled into the original peptide substrate.
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   Figure 3

   Mapping and functional prediction of MEN2 mutations targeting the RET catalytic domain. (A) Surface representations with common secondary structural and catalytic elements depicted in cartoon color coded. Glycine-rich loop (purple), αC helix (green), catalytic loop (orange), DFG motif (magenta), activation loop (yellow), P + 1 loop (wheat) showing in blue common residues mutated in MEN2. Cartoon representation of: (B) phosphorylated RET catalytic domain with two conformations of the glycine-rich loop and ATP-analogue. The closed glycine-rich loop conformer (non-competent for ATP binding) is defined by a tether between residues E734, R912 and D771, in addition to F735ⁱⁿ and E768^out rotamers, whereas the open (ATP competent) glycine-rich loop is defined by F735^out and E768ⁱⁿ rotamers. (C) RET N-lobe and hydrophobic motif showing β4 L790 as part of the R-spine and hydrophobic pocket. Ret L790 is caped from the top by W717 and points to αC helix F776 (i.e. hydrophobic pocket) and K789, and to the catalytically required K758–E775 pair. (D) Hinge region of RET between the N- and C-lobes depicting the β5 the gate-keeper mutation V804M. This residue can restrict nucleotide access to the active site, which in the case of RET substitution by a bulkier methionine (V804M) or even a leucine (V804L) is associated with resistance to several type I (DFGⁱⁿ) TKIs (e.g. vandetanib), while retaining nucleotide accessibility.

• © 2018 Society for Endocrinology