Biological significance of structural differences between two highly conserved Ubc variants.

Ubiquitin conjugating enzyme variants (Uev) Uev1 and Mms2 share >90% sequence identity but with distinct biological functions. Here, we report the monomeric and heterodimeric crystal structures of Uev1 and comparison with that of Mms2. Uev1 alone or in complex with Ubc13 is nearly identical with the corresponding Mms2 structures, except in one surface area containing 7/14 amino acid variations. To probe the biological significance of this unique region, we raised monoclonal antibodies specifically recognizing this region of Uev1, but not of Mms2. Epitope mapping and site-specific mutagenesis revealed at least two distinct epitopes within this region. These data collectively suggest the existence of cellular proteins capable of distinguishing Uev1 from Mms2 and directing the Ubc13–Uev complex to different pathways.