PML-II recruits ataxin-3 to PML-NBs and inhibits its deubiquitinating activity

Promyelocytic leukemia protein (PML) nuclear bodies (NBs) are dynamic and multiprotein complexes implicated in a variety of important biochemical events. Due to alternative mRNA splicing, PML has at least six nuclear isoforms that share a common N-terminus but differ in their C-terminal regions. However, the unique role of each PML isoform is not clear. Here, we report the characterization of the deubiquitinase ataxin-3 as a specific binding partner of PML isoform II (PML-II). Ataxin-3 was identified as a potential binding protein of PML-II in a yeast-hybrid screen employing the unique C-terminal region of PML-II as bait. Ataxin-3 only binds to the C-terminal region of PML-II and not that of other PML isoforms. The interaction between ataxin-3 and PML-II was confirmed by co-immunoprecipition assays, and immunofluorescent microscopy revealed that PML-II and ataxin-3 were co-localized in PML-NBs. In addition, PML-II not only interacts with ataxin-3 with a normal range of poly-Q repeats (13Q), but also with a pathological form of ataxin-3 with extended poly-Q repeats (79Q). Importantly, the deubiquitinase activity of ataxin-3 was inhibited by PML-II. Our results suggest that PML-II may be a negative regulator of ataxin-3.