The Pso4 mRNA splicing and DNA repair complex interacts with WRN for processing of DNA interstrand cross-links

DNA interstrand cross-links (ICLs) are perhaps the most formidable lesion encountered by the cellular DNA repair machinery, and the elucidation of the process by which they are removed in eukaryotic cells has proved a daunting task. In particular, the early stages of adduct recognition and uncoupling of the cross-link have remained elusive principally because genetic studies have not been highly revealing. We have developed a biochemical assay in which processing of a DNA substrate containing a site-specific psoralen ICL can be monitored in vitro. Using this assay we have shown previously that the mismatch repair factor MutSbeta, the nucleotide excision repair heterodimer Ercc1-Xpf, and the replication proteins RPA and PCNA are involved in an early stage of psoralen ICL processing. Here, we report the identification of two additional factors required in the ICL repair process, a previously characterized pre-mRNA splicing complex composed of Pso4/Prp19, Cdc5L, Plrg1, and Spf27 (Pso4 complex), and WRN the protein deficient in Werner syndrome. Analysis of the WRN protein indicates that its DNA helicase function, but not its exonuclease activity, is required for ICL processing in vitro. In addition, we show that WRN and the Pso4 complex interact through a direct physical association between WRN and Cdc5L. A putative model for uncoupling of ICLs in mammalian cells is presented.