THE HUMAN UBIQUITIN CARRIER PROTEIN E2(MR = 17 000) IS HOMOLOGOUS TO THE YEAST DNA-REPAIR GENE RAD6

Components of the ubiquitin conjugating system were purified from human placenta by covalent affinity chromatography on ubiquitin sepharose. In contrast to E2 preparations obtained from rabbit reticulocytes and erythrocytes or Saccharomyces cerevisiae, the placental E2 preparation lacks E2(M(r) = 14000) and E2(M(r) = 20000) which are both unique in catalysing the ligase-independent transfer of ubiquitin to histones. A novel technique was employed to detect ubiquitin carrier function of the E2 proteins after SDS-electrophoresis and blotting to nitrocellulose. A cDNA of E2(M(r) = 17000) was isolated from a human cDNA library by screening with a degenerate oligonucleotide whose sequence was based on a partial amino acid sequence obtained from an E2(M(r) = 17000) peptide. Sequence analysis demonstrated an identity of 69% in the primary sequence of human E2(M(r) = 17000) and the protein encoded by the yeast DNA repair gene RAD6, which was recently shown to be an E2 species in yeast. Such a high degree of similarity between the human E2(M(r) = 17000) and the yeast DNA repair enzyme is suggestive of important common structural constraints or roles in addition to ubiquitin carrier activity, since in yeast this function itself is not necessarily dependent on high conservation of primary structure.