Structure and Activity of the Peptidyl-Prolyl Isomerase Domain from the Histone Chaperone Fpr4 toward Histone H3 Proline Isomerization

The FK506-binding protein (FKBP) family of peptidyl-prolyl isomerases (PPIases) is characterized by a common catalytic domain that binds to the inhibitors FK506 and rapamycin. As one of four FKBPs within the yeast Saccharomyces cerevisiae , Fpr4 has been described as a histone chaperone, and is in addition implicated in epigenetic function in part due to its mediation of cis-trans conversion of proline residues within histone tails. To better understand the molecular details of this activity, we have determined the solution structure of the Fpr4 C-terminal PPIase domain by using NMR spectroscopy. This canonical FKBP domain actively increases the rate of isomerization of three decapeptides derived from the N terminus of yeast histone H3, whereas maintaining intrinsic cis and trans populations. Observation of the uncatalyzed and Fpr4-catalyzed isomerization rates at equilibrium demonstrate Pro 16 and Pro 30 of histone H3 as the major proline targets of Fpr4, with little activity shown against Pro 38 . This alternate ranking of the three target prolines, as compared with affinity determination or the classical chymotrypsin-based fluorescent assay, reveals the mechanistic importance of substrate residues C-terminal to the peptidyl-prolyl bond. Background: The yeast histone chaperone Fpr4 harbors a peptidyl-prolyl isomerase domain of the FK506-binding protein (FKBP) family. Results: Catalytic efficiency toward three peptides from histone H3 relates to residues flanking the central proline. Conclusion: Substrate residues C-terminal to the proline dictate isomerase activity by Fpr4. Significance: The findings reveal new molecular details of substrate peptide recognition by the peptidyl-prolyl isomerase domain.