Function and subcellular location of Ro52beta.

Autoantigen Ro52alpha was recently identified as an E3 ubiquitin ligase. Its splicing variant Ro52beta, which lacks a leucine zipper, has not been characterized yet. We therefore characterized Ro52beta in contrast to Ro52alpha. Our biochemical assays revealed that both Ro52alpha and Ro52beta function as E3 ubiquitin ligases and self-ubiquitinate in cooperation with UbcH5B in vitro. In addition, both Ro52alpha and Ro52beta 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 Ro52alpha mainly localizes to the cytoplasmic rod-like structures, whereas Ro52beta diffusely localizes to both the cytoplasm and the nucleus. Since the leucine zipper plays a role in the homodimerization and heterodimerization of Ro52alpha, the dimerization might be required for the localization of Ro52alpha to the rod-like structures. On the basis of these results, Ro52alpha and Ro52beta appear to ubiquitinate their particular substrates at different locations.