tokens: These results suggest that UBP41 may play an important role in the recycling of ubiquitin by hydrolysis of branched poly - ubiquitin chains generated by the action of 26 S proteasome on poly - ubiquitinated protein substrates , as well as in the production of free ubiquitin from linear poly - ubiquitin chains and of certain ribosomal proteins from ubiquitin fusion proteins .