Ubiquitin-binding protein that specifically binds 'Lys-6'-linked polyubiquitin chains. 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. Involved in ubiquitin-proteasome systems (By similarity). The UBA domain specifically recognizes and binds 'Lys-6'-linked polyubiquitin chains.