CGI-55 interacts with nuclear proteins and co-localizes to p80-coilin positive-coiled bodies in the nucleus.

The human protein CGI-55 has been described as a chromo-helicase-DNA-binding domain protein (CHD)-3 interacting protein and was also found to interact with the 3'-region of the plasminogen activator inhibitor (PAI)-1 mRNA. Here, we used CGI-55 as a "bait" in a yeast two-hybrid screen and identified eight interacting proteins: Daxx, Topoisomerase I binding RS (Topors), HPC2, UBA2, TDG, and protein inhibitor of activated STAT (signal transducer and activator of transcription) (PIAS)-1, -3, and -y. These proteins are either structurally or functionally associated with promyelocytic leukemia nuclear bodies (PML-NBs), protein sumoylation, or the regulation of transcription. The interactions of CGI-55 with Daxx, Topors, PIASy, and UBA2 were confirmed by in vivo colocalization experiments in HeLa cells, by using green (GFP) and red fluorescence fusion proteins. A mapping study of the CGI-55 binding site for these proteins revealed three distinct patterns of interaction. The fact that CGI-55-GFP has been localized in cytoplasm and nucleus in a dotted manner, and its interaction with proteins associated with PML-NBs, suggested that CGI-55 might be associated with nuclear bodies. Although Daxx and Topors co-localized with promyelocytic leukemia protein (PML), CGI-55 itself as well as PIASy and UBA2 showed only little co-localization with PML. However, we observed that CGI-55 localizes to the nucleolus and co-localizes with p80-coilin positive nuclear-coiled bodies.