Rothmund Thomson Syndromes (RTS) is a rare disease associated with genome instability, predisposition to cancer, skin and skeletal abnormalities, and some features of premature aging. The disease is caused by mutation in RECQL4, a putative helicase of the RecQ family that also includes WRN and BLM, proteins defective in Werner and Bloom syndromes, respectively. To understand the mechanism of this disease, we have purified a human RECQL4 complex and identified its associated components. We found that the bulk of RECQL4 was present in cytoplasmic extracts of HeLa cells, in contrast to the largely nuclear BLM and WRN helicases. However, in untransformed WI-38 fibroblasts, RECQL4 was found to be largely nuclear, and was present at significantly lower total levels than in transformed HeLa cells. RECQL4 from HeLa cells was isolated as a stable complex with UBR1 and UBR2. These 200 kD proteins are ubiquitin ligases of the N-end rule pathway, whose substrates include proteins with destabilizing N-terminal residues. The functions of this proteolytic pathway include the regulation of peptide import, chromosome stability, meiosis, apoptosis, and cardiovascular development. Although the known role of UBR1 and UBR2 is to mediate polyubiquitylation (and subsequent degradation) of their substrates, the UBR1/2-bound RECQL4 was not ubiquitylated in vivo, and was a long-lived protein in HeLa cells. The isolated RECQL4-UBR1/2 complex had a DNA-stimulated ATPase activity but was inactive in DNA-based assays for helicases and translocases, the assays in which the BLM helicase was active. Our data suggest that RECQL4 and ubiquitin ligases of the N-end rule pathway may play a role in maintaining genomic stability. ? We have also started purification and characterization of RECQL5, another RecQ helicase involved in maintaining genome stability in mammalian cells. Mutation of this gene has been shown to result in genomic instability in cells. We have so far purified a complex containing RECQL5 and identified its components. We have mapped their interaction domains, and are investigating if the interactions between RECQL5 and its associated components are required for RECQL5 to protect genome stability. ? In a study of the origin of DNA damage, we participated in a collaborative project to study how mitochondria produce reactive oxygen that can modify DNA.
