Human RecQ5 beta, a large isomer of RecQ5 DNA helicase, localizes in the nucleoplasm and interacts with topoisomerases 3 alpha and 3 beta

The RecQ helicase superfamily has been implicated in DNA repair and recombi nation. At least five human RecQ-related genes exist: RecQ1, BLM, WRN, RecQ 4 and RecQ5. Mlutations in BLM, WRN end RecQ4 are associated with Bloom, We rner and Rothmund-Thomson syndromes, respectively, involving a predispositi on to malignancies and a cellular phenotype that includes increased chromos ome instability. RecQ5 is small, containing only a core part of the RecQ he licase, but three isomer transcripts code for small RecQ5 alpha (correspond ing to the original RecQ5 with 410 amino acids), new large RecQ5 beta (991 amino acids) and small RecQ5 gamma (435 amino acids) proteins that contain the core helicase motifs, By determining the genomic structure, we found th at the three isoforms are generated by differential splicing from the RecQ5 gene that contains at least 19 exons, Northern blot analysis using a RecQ5 beta-specific probe indicates that RecQ5 beta mRNA is expressed strongly i n the testis, Immunocytochemical staining of three N-terminally tagged RecQ 5 isomers expressed in 293EBNA cells showed that RecQ5 beta migrates to the nucleus and exists exclusively in the nucleoplasm, while the small RecQ5a and RecQ5 gamma proteins stay in the cytoplasm, Immunoprecipitation and an extended cytochemical experiment suggested that the nucleoplasmic RecQ5 bet a, like yeast Sgs1 DNA helicase, binds to topoisomerases 3 alpha and 3 beta , but not to topoisomerase 1. These results predict that RecQ5 beta may hav e an important role in DNA metabolism and may also be related to a distinct genetic disease.