first Tudor domain found in ubiquitin-like PHD and RING finger domain-containing protein 1 (UHRF1) and similar proteins
UHRF1, also called inverted CCAAT box-binding protein of 90 kDa, nuclear protein 95, nuclear zinc finger protein Np95 (Np95), RING finger protein 106, transcription factor ICBP90, or E3 ubiquitin-protein ligase UHRF1, is a unique chromatin effector protein that integrates the recognition of both histone PTMs and DNA methylation. It is essential for cell proliferation and plays a critical role in the development and progression of many human carcinomas, such as laryngeal squamous cell carcinoma (LSCC), gastric cancer (GC), esophageal squamous cell carcinoma (ESCC), colorectal cancer, prostate cancer, and breast cancer. UHRF1 can act as a transcriptional repressor through its binding to histone H3 when it is unmodified at Arg2. Its overexpression in human lung fibroblasts results in downregulation of expression of the tumour suppressor pRB. It also plays a role in transcriptional repression of the cell cycle regulator p21. Moreover, UHRF1-dependent repression of factors can facilitate the G1-S transition. It interacts with Tat-interacting protein of 60 kDa (TIP60) and induces degradation-independent ubiquitination of TIP60. It is also a N-methylpurine DNA glycosylase (MPG)-interacting protein that binds MPG in a p53 status-independent manner in the DNA base excision repair (BER) pathway. In addition, UHRF1 functions as an epigenetic regulator that is important for multiple aspects of epigenetic regulation, including maintenance of DNA methylation patterns and recognition of various histone modifications. UHRF1 contains an N-terminal ubiquitin-like domain (UBL), a tandem Tudor domain (TTD), a plant homeodomain (PHD) domain, a SET- and RING- associated (SRA) domain, and a C-terminal RING-finger domain. It specifically binds to hemimethylated DNA, double-stranded CpG dinucleotides, and recruits the maintenance methyltransferase DNMT1 to its hemimethylated DNA substrate through its SRA domain. UHRF1-dependent H3K23 ubiquitylation has an essential role in maintenance DNA methylation and replication. The tandem Tudor domain directs UHRF1 binding to the heterochromatin mark histone H3K9me3 and the PHD domain targets UHRF1 to unmodified histone H3 in euchromatic regions. The RING-finger domain exhibit both autocatalytic E3 ubiquitin (Ub) ligase activity and activity against histone H3 and DNMT1. The model corresponds to the first Tudor domain.