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 → OC: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.