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 TIF1 ss, 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 TIF1 ss as a multilysine acceptor for SUMO which specifically targets six lysine residues (Lys(554), Lys(575), Lys(676), Lys(750), Lys(779), and Lys(804)) within the TIF1 ss C-terminal repressive region. Reporter gene assays indicate that TIF1 ss requires SUMO-modification for its repressive activity. Indeed, sumoylation-less mutants failed to recapitulate TIF1 ss-dependent repression. TIF1 ss 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 TIF1 ss sumoylation is a prerequisite for the recruitment of HDAC and that TIF1 ss 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, TIF1 ss sumoylation is positively regulated by its interaction with KRAB domain-containing proteins. Altogether, our results provide new mechanistic insights into TIF1 ss transcriptional repression and suggest that KRAB multifinger proteins not only recruit TIF1 ss co-repressor to target genes but also increase its repressive activity through enhancement of its sumoylation.