Tumor suppressor that is required to sustain G2/M checkpoint after DNA damage. Acts as a p53/TP53 activator by inhibiting MDM2 binding to p53/TP53 and stimulating non-proteolytic polyubiquitination of p53/TP53. Exhibits ubiquitin ligase (E3) activity and assemble ubiquitin polymers through 'Lys-11'- (K11-), 'Lys-29'- (K29-) and 'Lys-63'- (K63)-linkages, independently of the ubiquitin-conjugating enzyme (E2). Promotes p53/TP53-dependent transcription of CDKN1A/p21, leading to robust checkpoint response. Mediates CDKN1A/p21 protein stability in a ubiquitin-independent manner. Interacts with HDAC1 and prevents binding of HDAC1 to CDKN1A/p21 and facilitates the acetylation and stabilization of CDKN1A/p21 (By similarity). May have a role in the assembly of primary cilia (Probable). Interacts with HDAC1; the interaction prevents binding of HDAC1 to CDKN1A/p21 and facilitates the acetylation and stabilization of CDKN1A/p21. Interacts with p53/TP53; the interaction inhibits binding of p53/TP53 and MDM2. Localizes to the actin cytoskeleton in a proportion of cells. Colocalizes with centriolar acetylated tubulin. Truncated N-terminus.