A DNA Helicase Purified by Replication Protein a (RPA) Affinity Chromatography from Mouse FM3A Cells

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Abstract
In an effort to identify cellular helicases that mimic the action of SV40 large T-antigen, we performed replication protein A (RPA) affinity chromatography on cell extracts from the mouse mammary carcinoma cell line FM3A. In this way, a novel DNA helicase was isolated and purified to near homogeneity. The most purified fractions showed the presence of two proteins of 28 and 21 kDa. Both proteins interacted with 32 P-labeled partially duplex DNA when bound to nitrocellulose membranes and were efficiently UV crosslinked to [α- 32 P]dATP. Helicase activity was strongly stimulated by RPA on DNA substrates containing duplex regions longer than 18 bp. Only weak stimulation was observed in the presence of Escherichia coli single strand DNA binding protein (SSB). The enzyme unwinds DNA in the 5′-3′ direction in relation to the strand to which it binds. Only ATP and dATP were efficient as nucleoside triphosphate co-factors, and showed similar K m values of ∼0.6 mM. The properties of this enzyme suggest that it may take part in reactions mediated by RPA such as those predicted to occur at replication forks or alternatively may function during DNA repair or recombination.