Function and subcellular location of Ro52β

Autoantigen Ro52α was recently identified as an E3 ubiquitin ligase. Its splicing variant Ro52β, which lacks a leucine zipper, has not been characterized yet. We therefore characterized Ro52β in contrast to Ro52α. Our biochemical assays revealed that both Ro52α and Ro52β function as E3 ubiquitin ligases and self-ubiquitinate in cooperation with UbcH5B in vitro. In addition, both Ro52α and Ro52β are ubiquitinated when overexpressed with ubiquitin in HEK293T cells, suggesting that both function as E3 ligases and self-ubiquitinate in vivo. However, cytological studies revealed that Ro52α mainly localizes to the cytoplasmic rod-like structures, whereas Ro52β diffusely localizes to both the cytoplasm and the nucleus. Since the leucine zipper plays a role in the homodimerization and heterodimerization of Ro52α, the dimerization might be required for the localization of Ro52α to the rod-like structures. On the basis of these results, Ro52α and Ro52β appear to ubiquitinate their particular substrates at different locations.