The retinoblastoma interacting zinc finger gene RIZ produces a PR domain-lacking product through an internal promoter.

The PR domain is a newly recognized protein motif that characterizes a subfamily of Krüppel-like zinc finger genes. Members of the PR domain family have been shown to play important roles in cell differentiation and malignant transformation. The RIZ gene is the founding member of this family; it was isolated because its gene products can bind to the retinoblastoma tumor suppressor protein. Here, we have studied the RIZ gene structure and expression. By immunoprecipitation and immunoblot analysis we identified two different RIZ protein products of 280 and 250 kDa, designated RIZ1 and RIZ2, respectively. The 280-kDa RIZ1 product comigrated with the RIZ cDNA-derived polypeptide. The 250-kDa RIZ2 product lacked the NH2-terminal PR domain of RIZ1; it comigrated with a truncated RIZ1 polypeptide that was initiated from an internal ATG codon. Both the full-length and the truncated RIZ1 polypeptide were located in the nucleus as shown by transfection and immunofluorescence analysis. We identified the RIZ2 transcripts and showed that they were produced by an internal promoter located at the 5' boundary of coding exon 5. RNase protection analysis revealed similar ratios of RIZ1 and RIZ2 transcripts in most adult rat tissues except in testis, where RIZ1 was more abundant than RIZ2. These observations were strikingly similar to those described for the MDS1-EVI1 cancer gene, which also normally gives rise to a PR domain-lacking product, EVI1, because of an internal promoter.