A novel role of XRCC1 in the functions of a DNA polymerase beta variant

In the base excision repair pathway, wild-type DNA polymerase beta (WT polbeta) provides most of the gap filling synthesis. A truncated polbeta protein (polbetaDelta), expressed in primary colorectal and breast tumors and in a primary culture of renal cell carcinoma, inhibits the gap filling synthesis and DNA binding activities of WT polbeta. However, a purified recombinant polbetaDelta does not inhibit a purified WT polbeta. To determine the dominant inhibitory activity of polbetaDelta, we examined interactions of purified polbetaDelta with X-ray cross complementing group 1 (XRCC1), poly(ADP-ribose) polymerase (PARP), and apurinic endonuclease (Ape) proteins. All of these proteins interact with polbetaDelta in vitro and in vivo. The polbetaDelta protein can fill one nucleotide gap by inserting a base at the AP site, whereas a presumed binary complex of polbetaDelta and XRCC1 cannot. However, this binary complex not only suppresses gap filling synthesis activity of WT polbeta but also binds more strongly to gapped DNA than WT polbeta bound to XRCC1. These results are the first to suggest that XRCC1 is directly involved in the dominant negative activity of truncated polbeta, possibly leading to the genomic instability characteristic of tumor cells.