Abstract BARD1 (BRCA1-Associated Ring Domain 1) was originally identified in a yeast-two-hybrid (Y2H) screen as a binding partner of BRCA1. The functional heterodimer BRCA1/BARD1 is required for several of the cellular and tumor-suppressor functions of BRCA1. Both proteins interact through the N-terminal RING domain to form a heterodimeric E3 ubiquitin ligase that constitutes the major catalytic activity of the BRCA1-BARD1 complex. BARD1 is also associated to p53-mediated apoptosis, in a BRCA1-independent manner. The carboxy terminus of BRCA1 is highly acidic and contains two tandem BRCA1 C-terminal (BRCT) domains, which are characteristic of members of a large superfamily of proteins involved in DNA repair and cell cycle checkpoint control. These domains are protein-protein interacting regions that mediate the association with a number of other proteins with a role in DNA replication, DNA damage repair pathway, transcription, cell cycle control, and ubiquitination. Although several BRCA1-interacting proteins were reported, few proteins were described to interact with BARD1. In order to identify putative interaction-proteins with BARD1 C-terminal region we performed a Y2H assay using BARD1 BRCT tandem domain (aa 554-777) to screen a human testis cDNA library. We identified, among other hits, a cDNA coding for BCCIP (BRCA2 and CDKN1A Interacting Protein) C-terminal region. BCCIP is known for its involvement in DNA damage repair and the cell cycle control. BCCIP interacts with BRCA2, and both proteins are important for RAD51 focus formation after ionizing radiation and homologous recombination (HR) repair of double-strand-breaks (DSBs). BCCIP enhances the p21 suppression activity towards CDK2, and BCCIP downregulation reduces p21 expression by abrogating p53 transcription activity. We confirmed BCCIP/BARD1 interaction by pull-down and co-immunoprecipitation (co-IP) analysis of ectopically expressed BARD1 and BCCIP. We also identified the constitutive BCCIP/BARD1 complex in human cells by co-IP assays. Interestingly the BCCIP observed in complex with BARD1 corresponds to a mono-ubiquitinated form, suggesting that BCCIP may be a putative natural target for BRCA1/BARD1 heterodimer E3 ubiquitin ligase. BRCA1 participation in a multicomponent complex with BCCIP is under investigation. In order to assess the biological significance of the interaction between BCCIP and BARD1 we are investigating its role in DNA damage repair, cell cycle control and apoptosis. We are also evaluating the BCCIP protein interaction network using a tandem affinity purification approach. Citation Format: Joao Séllos, Thales C. Nepomuceno, Renato S. Carvalho, Guilherme Suarez-Kurtz, Alvaro N.A. Monteiro, Marcelo A. Carvalho. Characterization of BCCIP as a novel BARD1 interaction partner. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 632. doi:10.1158/1538-7445.AM2013-632