Ubiquitin-binding protein that interacts with the BRCA1-BARD1 heterodimer, and regulates its activity. Specifically binds 'Lys-6'-linked polyubiquitin chains. Interaction with autoubiquitinated BRCA1, leads to inhibit the E3 ubiquitin-protein ligase activity of the BRCA1-BARD1 heterodimer. Component of a complex required to couple deglycosylation and proteasome-mediated degradation of misfolded proteins in the endoplasmic reticulum that are retrotranslocated in the cytosol (By similarity). Component of a complex required to couple retrotranslocation, ubiquitination and deglycosylation composed of NGLY1, SAKS1, AMFR, VCP and RAD23B. Interacts with HOMER2 (By similarity). Interacts directly with VCP. Interacts with BRCA1 and BARD1; interaction takes place when BRCA1 is not autoubiquitinated bur is strongly enhanced in the presence of autoubiquitinated BRCA1 (By similarity). The UBA domain specifically recognizes and binds 'Lys-6'-linked polyubiquitin chains.