Sir4 H-BRCT domain interacts with multiple phospho-proteins to regulate heterochromatic repression

In Saccharomyces cerevisiae, the Silent Information Regulator (SIR) proteins Sir2/3/4 form a complex suppressing transcription of genes at subtelomeric regions and the homothallic mating type (HM) loci. Here we identify a non-canonical BRCA1 C-terminal domain (H-BRCT) in Sir4 which is responsible for tethering telomeres to the nuclear periphery. We show that Sir4 H-BRCT and the closely related Dbf4 H-BRCT serve as selective phospho-epitope recognition domains that bind to a variety of phosphorylated target peptides. We present detailed structural information about the binding mode of established Sir4 interactors (Esc1, Ty5, Ubp10) and identify several novel interactors of Sir4 H-BRCT, including the E3 ubiquitin ligase Tom1. Based on these findings, we propose a phospho-peptide consensus motif for interaction with Sir4 H-BRCT and Dbf4 H-BRCT. Ablation of the Sir4 H-BRCT phospho-peptide interaction disrupts SIR-mediated repression and perinuclear localization. In conclusion, the Sir4 H-BRCT domain serves as a hub for recruitment of phosphorylated target proteins to heterochromatin to properly regulate silencing and nuclear order.