Joint surveillance of the replication foci by PCNA and CtIP.

CtIP (CtBP-interacting protein) is emerging as a key new player in the maintenance of genome stability. CtIP was discovered as a cofactor of the ubiquitous transcriptional corepressor CtBP.1 by virtue of its association with CtBP, it is presumed to function in transcriptional regulation. CtIP also interacts with the pRb family proteins and BRCA1 and has been linked to the transcriptional repression activities mediated by these tumor suppressors. While interaction of CtIP with pRb appears to be important for G1/S regulation,2 CtIP activity is also required for G2/M DNA damage checkpoint via transient association with BRCA1 and Chk1 activation during treatment with ionizing radiation.3 Recently, orthologs of CtIP that participate in DNA double strand repair have been identified in yeast, fungi, worm and plants.4–7 Specifically, CtIP and its orthologs have been shown to be critical for dsDNA break repair by homologous recombination. These orthologs share a distinct region of homology with the C-terminal region of CtIP. CtIP and its orthologs have been shown to associate with the Mre11/Rad50/NSB1 (MRN) complex and mediate DNA end resection to generate ssDNA intermediates for recombination. While CtIP orthologs appear to function primarily in DNA repair, the vertebrate CtIP appear to be a multifunctional modular adapter protein recruiting multiple protein complexes to mediate concerted cell cycle regulation and DNA damage checkpoint control and DNA repair. In the current issue of Cell Cycle, Gu and Chen8 report that CtIP is targeted to DNA replication foci by direct interaction with PCNA. The interaction between PCNA and CtIP is mediated by a 42-amino acid region of CtIP known as Replication Foci Targeting Sequence (RFTS) that contains a consensus PCNA-interacting motif.9 Endogenous CtIP and that expressed by transient transfection colocalized with PCNA in replication foci and mutations in the RFTS abolished such colocalization. Interfering with the interaction between CtIP and PCNA via overexpression of an RFTS-GFP chimeric construct (dominant negative) induced cell cycle arrest at S/G2 transition and cessation of cell proliferation. Cells expressing the chimeric RFTS construct induced DNA damage and activation of DNA damage checkpoint while a mutant RFTS construct defective in interaction with PCNA did not. These results suggest that the CtIP-PCNA complex might play a role in stabilization of the stalled replication forks. Since CtIP plays a role in DNA repair, it is possible that the normal function of the PCNA-CtIP complex might be involved in surveillance of the replication forks for potential stalling. Considering the established role of CtIP in DNA repair and the present results that PCNA directs CtIP to the replication foci, new investigations on the role CtIP in repair of DNA damage at the replication fork should be forthcoming.