BRCA1-associated protein 1 (BAP1) is a ubiquitin COOH-terminal hydrolase that was initially identified as a protein that binds the RING finger domain of the breast and ovarian tumor suppressor BRCA1. BAP1 is a tumour suppressor that is believed to mediate its effects through chromatin modulation, transcriptional regulation, and possibly via the ubiquitin-proteasome system and the DNA damage response pathway (Murali et al. 2013).

The C-terminal coiled coil motif of BAP1 directly interacts with the zinc fingers of the transcription factor Yin Yang 1 (YY1) (Yu et al. 2010), part of a multiprotein complex containing numerous transcription factors and cofactors including the transcriptional regulator Host cell factor 1 (HCFC1), which binds the N-terminal portion of BAP1 (Misaghi et al. 2009, Machida et al. 2009). HCFC1 is a chromatin-associated protein initially identified as part of a multiprotein complex comprising the viral coactivator VP16 and the POU domain transcription factor POU2F1. During herpes simplex virus infection, this complex is recruited to the enhancer/promoter of the immediate-early gene to activate viral gene expression (Kristie et al. 2010).

The C-terminal extension of UCHL5 mediates association with Adrm1/Rpn13 of the proteasomal 19S regulatory subunit and with NFRKB of the INO80 chromatin remodeling complex. The extreme C-terminal segment of BAP1 is 38% identical to the C-terminus of UCHL5 (UCH37) and is necessary for binding to YY1 (Yu et al. 2010).