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Figure 3
Secondary and tertiary protein–protein interfaces in C-DCX. (a) A Lys271-Pro272 cis-prolyl peptide bond is present at the secondary interface generated in the domain-swapped dimer. The cis-­peptide had previously gone unnoticed owing to a lack of clear electron density in the monomeric C-DCX crystal structure (PDB entry 5ip4) but is well defined in this structure (2FoFc density shown as a blue mesh contoured at 1 r.m.s.d.). The backbone amides of this cis-peptide form reciprocal hydrogen bonds across the secondary interface (black dashed lines). A large symmetric tertiary interface of ∼2700 Å2 centred at a twofold axis (marked) connects two domain-swapped dimers into a tetramer. The scheme on the lower right shows that both subunits of each dimer partake in this interface. Difference OMIT electron density contoured at 3 r.m.s.d. indicates a ligand bound at the top of the interface. (b) The stereoview of the tertiary interface after rotation by 90° about the horizontal axis shows a small hydrophobic core formed by aliphatic side chains below the difference density. The bottom part of the interface is dominated by polar interactions, including Arg277, which is mutated to histidine in lissencephaly. (c) The electron density can be modelled by a peptide. The sequence PESSEG was chosen, but is tentative. The two possible orientations of the peptide are shown as thick green and thin black sticks. The carboxylate groups (Glu and the C-terminus) of the peptide are in hydrogen-bond geometry with the guanidinium side chain of Arg273 and the main-chain amide NH group of Gly269. The proline chosen to model the electron-density stacks on top of the phenolic side chain of Tyr310. In the other orientation, glycine stacks on Tyr310. Amino-acid names are given for the peptide orientation shown in thick sticks.

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ISSN: 2059-7983
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