A human homologue of the Schizosaccharomyces pombe rad1+ checkpoint gene encodes an exonuclease

In the fission yeast Schizosaccharomyces pombe the rad1(+) gene is required for both the DNA damage-dependent and the DNA replication-dependent cell cycle checkpoints. We have identified a human homologue of the S. pombe rad1(+) gene, designated Hrad1, as well as a mouse homologue: Mrad1. Two Hrad1 alternative splice variants with different open reading frames have been identified; one codes for a long form, Hrad1A, and the other encodes a short form because of N-terminal truncation, Hrad1B. Hrad1A has 60% identity to the S. pombe rad1+ sequence at the DNA level and 49% identity and 72% similarity at the amino acid level. Northern blot analysis indicates elevated levels of expression in testis and Cancer cell lines. Chromosomal localization by fluorescence in situ hybridization indicates that Hrad1 is located on chromosome 5p13. 2-13.3. This region is subject to loss of heterozygosity in several human cancers. Hrad1 also shares homology with the Saccharomyces cerevisiae RAD17 and Ustilago maydis REC1 proteins. REC1 has previously been characterized as a 3' --> 5' exonuclease with a C-terminal domain essential for cell cycle checkpoint function. We have expressed and purified polyhistidine-tagged fusions of Hrad1A and Hrad1B and show that HisHrad1A has 3' --> 5' exonuclease activity, whereas HisHrad1B lacks such activity. The biological functions of the two proteins remain to be determined.