Synonyms: Background:May catalyze the cis-trans isomerization of proline imidic peptide bonds in oligopeptides thereby assisting the folding of proteins(By similarity). May also function as a chaperone, playing a role in transport to the cell membrane of BSG for instance(PubMed:15946952). May also have a protein ubiquitin ligase activity acting as an E3 ubiquitin protein ligase or as an ubiquitin-ubiquitin ligase promoting elongation of ubiquitin chains on substrates. By mediating 'Lys-48'-linked polyubiquitination of proteins could target them for proteasomal degradation(PubMed:11435423).