Sumoylation of the Transcriptional Intermediary Factor 1β (TIF1β), the Co-repressor of the KRAB Multifinger Proteins, Is Required for Its Transcriptional Activity and Is Modulated by the KRAB Domain

Small ubiquitin-related modifier (SUMO) has emerged as a key post-translational modulator of protein functions. Here we show that TIF1β, a developmental regulator proposed to act as a universal co-repressor for the large family of KRAB domain-containing zinc finger proteins, is a heavily SUMO-modified substrate. A combined analysis of deletion and punctual mutants identified TIF1β as a multilysine acceptor for SUMO which specifically targets six lysine residues (Lys554, Lys575, Lys676, Lys750, Lys779, and Lys804) within the TIF1β C-terminal repressive region. Reporter gene assays indicate that TIF1β requires SUMO-modification for its repressive activity. Indeed, sumoylation-less mutants failed to recapitulate TIF1β-dependent repression. TIF1β homodimerization properties and interaction with the KRAB domain are preserved in the mutants with lysine to arginine substitutions as confirmed by in vivo bioluminescence resonance energy transfer (BRET). Using histone deacetylase (HDAC) inhibitors, we also demonstrate that TIF1β sumoylation is a prerequisite for the recruitment of HDAC and that TIF1β SUMO-dependent repressive activity involves both HDAC-dependent and HDAC-independent components. Finally, we report that, in addition to relying on the integrity of its PHD finger and on its self-oligomerization, TIF1β sumoylation is positively regulated by its interaction with KRAB domain-containing proteins. Altogether, our results provide new mechanistic insights into TIF1β transcriptional repression and suggest that KRAB multifinger proteins not only recruit TIF1β co-repressor to target genes but also increase its repressive activity through enhancement of its sumoylation.