endogenous SUMO-1. Both ataxin-3-20Q and ataxin-3-20QK166R were localized in the nucleus and cytoplasm uniformly, and the aggregates that formed expressed ataxin-3-68Q and ataxin-3-68QK166R (A). Immunoblotting analysis of subcellular fractionation of ataxin-3 shows no differences between the various groups (B). doi:10.1371/journal.pone.0054214.gsame lysine residue as ubiquitin, many researches have revealed a dynamic interplay between the related ubiquitination and SUMOylation pathways [38]. We first performed immunoprecipitation assays to detect the ubiquitination differences between ataxin-3 
and ataxin-3K166R. However, we didn’t find any evidence that SUMOylation of ataxin-3 affect ataxin-3 ubiquitination, which also indicate there is no competition between SUMO-1 and ubiquitin for binding site K166. Subsequentl.Tagged proteins were enriched with NTA 1516647 magnetic nickel columns and detected by immunoblotting with SUMO-1 antibody. **, ataxin-3-20Q modified by SUMO-1, ##, ataxin-3-68Q modified by SUMO-1. doi:10.1371/journal.pone.0054214.gleast on five and huntingtin on three lysine residues [26?7]. SCA3/MJD is the most common spinocerebellar ataxia diseases. In our previous research, we found that ataxin-3 was also a substrate of SUMO-1 [32]. In order to identify the motif residue, mutagenesis analyses were carried out to converse lysine 166 residues to arginine, which lies within a SUMO consensus sequence, VKGD, in ataxin-3. This conversion completely blocked the SUMOylation of ataxin-3. However, the conversion of other lysines, K8 and K206, which also lie within the SUMO consensus sequence in ataxin-3, did not affect SUMOylation of ataxin-3. These data suggest that K166 in ataxin-3 is the major SUMOylation binding site. Modification by SUMO has been shown to play critical roles in subcellular localization, and protein degradation, which ultimately contribute to regulation of the cell cycle, cell growth, and apoptosis [37]. In order to examine whether SUMOylation of ataxin-3 affects its subcellular localization, we compared the localization of ataxin-3 in transiently transfected HEK293 cells. In agreementwith previous studies, we found that the wild-type ataxin-3 protein was diffusively distributed in both nucleus and cytoplasm, while mutant-type ataxin-3 protein formed aggregates in nucleus. However, when we compared ataxin-3 and its SUMOylation deficient variant, we could not detect any difference in the subcellular localization of ataxin-3 in both immunofluorescent staining and immunoblot analysis, which indicates SUMOylation of ataxin-3 does not change its subcellular distribution. The similar result was also observed in SCA7, that SUMOylation on K257 of ataxin-7 does not influence its subcellular localization [28]. As we know, abnormal accumulation of mutant ataxin-3 in affected neurons reflexes that mutant protein may not be properly degraded. We found the insoluble fraction of ataxin-3-68Q was more than that of ataxin-3-20Q, which supported that mutanttype ataxin-3 protein was stable and easy to form aggregates. As SUMO modification of proteins is involved in protein degradation, it is possible that sumoylation of ataxin-3 may regulate its degradation process. Since SUMO-1 modifications target theThe Effect of SUMOylation on Ataxin-Figure 2. SUMO-1 modification did not affect the subcellular localization of ataxin-3. HEK293 cells were transfected with plasmids expressing GFP-tagged ataxin-3 or mutant ataxin-3K166R in the presence of endogenous SUMO-1. Both ataxin-3-20Q and ataxin-3-20QK166R were localized in the nucleus and cytoplasm uniformly, and the aggregates that formed expressed ataxin-3-68Q and ataxin-3-68QK166R (A). Immunoblotting analysis of subcellular fractionation of ataxin-3 shows no differences between the various groups (B). doi:10.1371/journal.pone.0054214.gsame lysine residue as ubiquitin, many researches have revealed a dynamic interplay between the related ubiquitination and SUMOylation pathways [38]. We first performed immunoprecipitation assays to detect the ubiquitination differences between ataxin-3 and ataxin-3K166R. However, we didn’t find any evidence that SUMOylation of ataxin-3 affect ataxin-3 ubiquitination, which also indicate there is no competition between SUMO-1 and ubiquitin for binding site K166. Subsequentl.