Identification of the protein Zibra, its genomic organization, regulation, and expression in breast cancer cells.

The mRNA that encodes zibra (zinc, in-between-ring finger, ubiquitin-associated domain), previously known as hypothetical protein FLJ10111, or RNF31 is expressed in several distinct cancers. Little is known about the genomic organization, expression, or regulation of zibra. Using RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE), we cloned the full-length zibra cDNA from a transformed breast cell line. We identified a novel exon, the 5' untranslated region including the +1 start site, and three alternatively spliced zibra transcripts. The zibra protein contains three zinc ring-finger motifs, an ubiquitin-associated domain, and an in-between-ring-finger domain, characteristic of ubiquitin ligases. We obtained an antibody to zibra and confirmed the presence of translated zibra protein for the first time. Promoter studies localized a core element responsible for basal activity to a 14-bp region in the 5' untranslated region. Although there are numerous consensus Ets factor binding sites in the zibra promoter, we found no affect on promoter activity from Ets-1, PDEF, or PEA-3/E1A-F. Treatment of cells with the proteasome inhibitor I (PSI) decreased zibra protein to an undetectable level after 8 h. Zibra remained undetectable even after 32 h, while mRNA levels remained essentially unchanged. In conclusion, zibra is a translationally regulated putative ubiquitin ligase that is frequently overexpressed in different forms of cancer.