sentence: The formation of this class of “ non canonical ” polyubiquitin chains is mostly catalyzed by the heterodimeric ubiquitin conjugating enzyme formed by Ubc13 and a Uev protein , Uev1 or Uev2 / Mms2 in higher eukaryotes , or Mms2 in the yeast S. cerevisiae [ 2 ] , [ 4 ] , [ 5 ] .