An homology region containing four conserved motifs has been identified in proteins from eukaryotes, several groups of viruses and the pathogenic bacteria Chlamydia pneumoniae [1]. None of these proteins has a known biochemical function but low sequence similarity with the polyprotein regions of arteriviruses has lead to suggest that it could possess cysteine protease activity [1]. In this case, the conserved cysteine and aspartate in motif I and the histidine in motif IV could be the catalytic residues. Motifs II and III have a more limited sequence conservation and could be involved in substrate recognition [1].

It has been proposed that the eukaryotic proteins containing an OTU domain could mediate proteolytic events involved in signaling associated with the modification of chromatin structure and control of cell proliferation [1].

Some proteins containing a OTU domain are listed below [1]:

• Ovarian Tumor (OTU), a Drosophila protein involved in oocyte morphogenesis. As the putative catalytic cysteine is replaced by a serine it is not clear whether OTU is an active protease or in inactivated protease homologue.
• Putative HIV-1 induced protein (HIN1), the human homologue of OTU.
• B0546.2 protein, the Caenorhabditis elegans homologue of OTU.
• M2 protein from La France disease virus.
• Tumor-necrosis-factor-α-induced protein A20 (TNP3) from mammals. It protects cells from apoptosis and blocks activation of lymphoid cells. In addition to the OTU domain, TNP3 contains several copies of a zinc finger like motif that is also present in other proteins implicated in ubiquitin- dependent proteolysis.
• Hypothetical protein YFL044C from yeast, a protein that also contains a C2H2 zinc finger.
• Hypothetical protein F12K2.7 protein from Arabidopsis thaliana.
• Hypothetical protein C25D7.8 protein from Caenorhabditis elegans.
• Hypothetical protein CPN0483 from Chlamydia pneumoniae.

The profile we developed covers the entire OTU domain.