A nuclear FK506-binding protein is a histone chaperone regulating rDNA silencing

We report a novel chromatin-modulating factor, nuclear FK506-binding protein (FKBP). It is a member of the peptidyl prolyl cis-trans isomerase (PPIase) family, whose members were originally identified as enzymes that assist in the proper folding of polypeptides. The endogenous FKBP gene is required for the in vivo silencing of gene expression at the rDNA locus and FKBP has histone chaperone activity in vitro. Both of these properties depend on the N-terminal non-PPIase domain of the protein. The C-terminal PPIase domain is not essential for the histone chaperone activity in vitro, but it regulates rDNA silencing in vivo. Chromatin immunoprecipitation showed that nuclear FKBP associates with chromatin at rDNA loci in vivo. These in vivo and in vitro findings in nuclear FKBPs reveal a hitherto unsuspected link between PPIases and the alteration of chromatin structure.