As the sole E2 enzyme for SUMOylation, Ubc9 is predominantly nuclear. However, 
the underlying mechanisms of Ubc9 nuclear localization are still not well 
understood. Here we show that RNAi-depletion of Imp13, an importin known to 
mediate Ubc9 nuclear import, reduces both Ubc9 nuclear accumulation and global 
SUMOylation. Furthermore, Ubc9-R13A or Ubc9-H20D mutation previously shown to 
interrupt the interaction of Ubc9 with nucleus-enriched SUMOs reduces the 
nuclear enrichment of Ubc9, suggesting that the interaction of Ubc9 with the 
nuclear SUMOs may enhance Ubc9 nuclear retention. Moreover, Ubc9-R17E mutation, 
which is known to disrupt the interaction of Ubc9 with both SUMOs and Imp13, 
causes a greater decrease in Ubc9 nuclear accumulation than Ubc9-R13A or 
Ubc9-H20D mutation. Lastly, Ubc9-K74A/S89D mutations that perturb the 
interaction of Ubc9 with nucleus-enriched SUMOylation-consensus motifs has no 
effect on Ubc9 nuclear localization. Altogether, our results have elucidated 
that the amino acid residues within the N-terminal region of Ubc9 play a pivotal 
role in regulation of Ubc9 nuclear localization.