Nucleolar localization and dynamic roles of flap endonuclease 1 in ribosomal DNA replication and damage repair

Departments of Radiation Biology, City of Hope National Medical Center and Beckman Research Institute, Duarte, CA 91010

^* To whom correspondence should be addressed. Email: bshen{at}coh.org.

	Abstract

Despite the wealth of information available on the biochemical functions and our recent findings of its roles in genome stability and cancer avoidance of the structure specific flap endonuclease-1 (FEN1), its cellular compartmentalization and dynamics corresponding to its involvement in various DNA metabolic pathways, are not yet elucidated. Several years ago we demonstrated that FEN1 migrates into the nucleus in response to DNA damage and under certain cell cycle conditions. In the current manuscript, we found that FEN1 is super-accumulated in the nucleolus and plays a role in resolution of stalled DNA replication forks formed at the sites of natural replication fork barriers. In response to UV irradiation and upon phosphorylation, FEN1 migrates to nuclear plasma to participate in resolution of UV cross-links on DNA, most likely employing its concerted action of exonuclease (EXO) and gap-dependent endonuclease (GEN) activities. Based on the yeast complementation experiments, mutation of Ser¹⁸⁷Asp, mimicking constant phosphorylation, excludes FEN1 from nucleolar accumulation. Replacement of the Ser187 by Ala, eliminating the only phosphorylation site, retains FEN1 in nucleoli. Both of the mutations cause UV sensitivity, impair cellular UV damage repair capacity, and decline overall cellular survivorship