Figure 1

Phosphopeptide interactions with the Nbs1 FHA domain. (A) Peptide-protein interface of the previously reported complex of the S. pombe Nbs1 FHA domain with a phosphorylated Ctp1 peptide (PDB: 3HUF⁹). The disordered sidechain of the Ctp1 N-terminal pSer⁷⁷ residue was modeled into the structure using PyMol (www.pymol.org). The pSer phosphate oxygens form H-bonds with Arg27, Asn28, and with the pSer backbone NH. (B) The peptide binding interface of the modeled hsNbs1 FHA domain. The initial peptide placement was obtained by overlaying the modeled structure with 3HUF and mutating the bound Ctp1 peptide to a generic SDT sequence present in MDC1. The pSer bridging oxygen is H-bonded to Arg28 and the non-bonding phosphate oxygens form H-bonds with Arg28, Lys29, and the pSer NH. The peptide pThr bridging oxygen is also H-bonded with Arg28, and the non-bridging oxygens form H-bonds with Ser42, Arg43 guanidino and backbone NH, and with the Lys73 amino sidechain. The model also shows extensive H-bonds with the peptide backbone (indicated by dashed black lines). Stick models of the Nbs1 FHA domains are shown in green, and the bound peptides in yellow. H-bond distances are summarized in Table S1.