Deubiquitinase with an ATP-independent isopeptidase activity, cleaving at the C-terminus of the ubiquitin moiety. Catalyzes its own deubiquitination. In vitro, isoform 2, but not isoform 3, shows deubiquitinating activity. Promotes plasma membrane localization of ARF6 and selectively regulates ARF6-dependent endocytic protein trafficking. Is able to initiate tumorigenesis by inducing the production of matrix metalloproteinases following NF-kappa-B activation. May act as a GTPase-activating protein for RAB3A (PubMed:19077034). {ECO:0000269|PubMed:15509780, ECO:0000269|PubMed:16127172, ECO:0000269|PubMed:19077034, ECO:0000269|PubMed:20418905}. This is the function of USP6 (ubiquitin specific peptidase 6, Ensembl gene identifier ENSG00000129204).