Template‐based docking of a prolactin receptor proline‐rich motif octapeptide to FKBP12: Implications for cytokine receptor signaling

A conserved proline‐rich motif (PRM) in the cytoplasmic domain of cytokine receptors has been suggested to be a signaling switch regulated by the action of the FK506 binding protein (FKBP) family of peptidylprolyl isomerases (O'Neal KD, Yu‐Lee LY, Shearer WT, 1995, Ann NY Acad Sci 766:282–284). We have docked the prolactin receptor PRM (Ile¹‐Phe²‐Pro³‐Pro⁴‐Val⁵‐Pro⁶‐Gly⁷‐Pro⁸) to the ligand binding site of FKBP12. The procedure involved conformational search restricted by NMR restraints (O'Neal KD et al., 1996, Biochem J 315:833–844), energy minimization of the octapeptide conformers so obtained, template‐based docking of a selected conformer to FKBP12, and energy refinement of the resulting complex. The template used was the crystal structure of a cyclic FK506‐peptide hybrid bound to FKBP12. Val⁵‐Pro⁶ of the PRM was taken to be the biologically relevant Xaa‐Pro bond. The docked conformer is stabilized by two intramolecular hydrogen bonds, N⁷H⁷ → O⁴ and N²H² → O⁸, and two intermolecular ones, Ile⁵⁶: N‐H → O=C:Pro⁶ and Tyr₈₂:O‐H → O=C:Gly⁷. This conformer features a Type I β‐turn and has extensive hydrophobic contacts with the FKBP12 binding surface. The observed interactions support the hypothesis that FKBP12 catalyzes cis‐trans isomerization in the PRM when it is part of the longer cytoplasmic domain of a cytokine receptor, and suggest a significant role for the PRM in signal transduction.