Phosphorylation at Ser473 Regulates Heterochromatin Protein 1 Binding and Corepressor Function of TIF1beta/KAP1

Background: As an epigenetic regulator, the transcriptional intermediary factor 1beta (TIF1beta)/KAP1/TRIM28) has been linked to gene expression and chromatin remodeling at specific loci by association with members of the heterochromatin protein 1 (HP1) family and various other chromatin factors. The interaction between TIF1beta and HP1 is crucial for heterochromatin formation and maintenance. The HP1-box, PXVXL, of TIF1beta is responsible for its interaction with HP1. However, the underlying mechanism of how the interaction is regulated remains poorly understood.

Results: This work demonstrates that TIF1beta is phosphorylated on Ser473, the alteration of which is dynamically associated with cell cycle progression and functionally linked to transcriptional regulation. Phosphorylation of TIF1beta/Ser473 coincides with the induction of cell cycle gene cyclin A2 at the S-phase. Interestingly, chromatin immunoprecipitation demonstrated that the promoter of cyclin A2 gene is occupied by TIF1beta and that such occupancy is inversely correlated with Ser473 phosphorylation. Additionally, when HP1beta was co-expressed with TIF1beta/S473A, but not TIF1beta/S473E, the colocalization of TIF1beta/S473A and HP1beta to the promoters of Cdc2 and Cdc25A was enhanced. Non-phosphorylated TIF1beta/Ser473 allowed greater TIF1beta association with the regulatory regions and the consequent repression of these genes. Consistent with possible inhibition of TIF1beta's corepressor function, the phosphorylation of the Ser473 residue, which is located near the HP1-interacting PXVXL motif, compromised the formation of TIF1beta-HP1 complex. Finally, we found that the phosphorylation of TIF1beta/Ser473 is mediated by the PKCdelta pathway and is closely linked to cell proliferation.

Conclusion: The modulation of HP1beta-TIF1beta interaction through the phosphorylation/de-phosphorylation of TIF1beta/Ser473 may constitute a molecular switch that regulates the expression of particular genes. Higher levels of phosphorylated TIF1beta/Ser473 may be associated with the expression of key regulatory genes for cell cycle progression and the proliferation of cells.