This ubiquitin C-terminal hydrolase (UCH) family includes UCH37 (also known as UCH-L5) and BRCA1-associated protein-1 (BAP1). They contain a UCH catalytic domain as well as an additional C-terminal extension which plays a role in protein-protein interactions. UCH37 is responsible for ubiquitin (Ub) isopeptidase activity in the 19S proteasome regulatory complex; it disassembles Lys48-linked poly-ubiquitin from the distal end of the chain. It is also associated with the human Ino80 chromatin-remodeling complex (hINO80) in the nucleus and can be activated through transient association of hINO80 with hRpn13 that is bound to the 19S regulatory particle or the proteasome. UCH37 possibly plays a role in oncogenesis; it competes with Smad ubiquitination regulatory factor 2 (Smurf2, ubiquitin ligase) in binding concurrently to Smad7 in order to deubiquitinate the activated type I transforming growth factor beta (TGF-beta) receptor, thus rescuing it from proteasomal degradation. BAP1 binds to the wild-type BRCA1 RING finger domain, localized in the nucleus. In addition to the UCH catalytic domain, BAP1 contains a UCH37-like domain (ULD), binding domains for BRCA1 and BARD1, which form a tumor suppressor heterodimeric complex, and a binding domain for HCFC1, which interacts with histone-modifying complexes during cell division. The full-length human BRCA1 is a ubiquitin ligase. However, BAP1 does not appear to function in the deubiquitination of autoubiquitinated BRCA1. BAP1 exhibits tumor suppressor activity in cancer cells, and gene mutations have been reported in a small number of breast and lung cancer samples. In metastasis of uveal melanoma, the most common primary cancer of the eye, inactivating somatic mutations have been identified in the gene encoding BAP1 on chromosome 3p21.1. These mutations include several that cause premature protein termination as well as affect its UCH domain, thus implicating loss of BAP1 and suggesting that the BAP1 pathway may be a valuable therapeutic target.
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