The Wnt/β-catenin signalling pathway plays essential roles in embryonic development and adult tissue homeostasis, and deregulation of this pathway has been linked to cancer. Axin is a concentration-limiting component of the β-catenin destruction complex, and its stability is regulated by tankyrase. However, the molecular mechanism by which tankyrase-dependent poly(ADP-ribosyl)ation (PARsylation) is coupled to ubiquitylation and degradation of axin remains undefined. Here, we identify RNF146, a RING-domain E3 ubiquitin ligase, as a positive regulator of Wnt signalling. RNF146 promotes Wnt signalling by mediating tankyrase-dependent degradation of axin. Mechanistically, RNF146 directly interacts with poly(ADP-ribose) through its WWE domain, and promotes degradation of PARsylated proteins. Using proteomics approaches, we have identified BLZF1 and CASC3 as further substrates targeted by tankyrase and RNF146 for degradation. Thus, identification of RNF146 as a PARsylation-directed E3 ligase establishes a molecular paradigm that links tankyrase-dependent PARsylation to ubiquitylation. RNF146-dependent protein degradation may emerge as a major mechanism by which tankyrase exerts its function.

The co-activators CBP and p300 are important for normal cell differentiation and cell cycle progression and are the targets for viral proteins that dysregulate these cellular processes. We show here that the E6 protein from the oncogenic human papillomavirus type 16 (HPV-16) binds to three regions (C/H1, C/H3 and the C-terminus) of both CBP and p300. The interaction of E6 with CBP/p300 was direct and independent of proteins known to bind the co-activators, such as p53. The E6 protein from low-risk HPV type 6 did not interact with C/H3 or the C-terminus but associated with the C/H1 domain at 50% of the level of HPV-16. HPV-16 E6 inhibited the intrinsic transcriptional activity of CBP/p300 and decreased the ability of p300 to activate p53-and NF-κB-responsive promoter elements. Interestingly, some mutations in HPV-16 E6 abrogated C/H3-E6 interactions, but did not alter the ability of E6 to associate with the C/H1 domain, suggesting that these modified proteins could be used to delineate the functional significance of the C/H1 and C/H3 domains of CBP/p300.