3′- to 5′ DNA unwinding by TIP49b proteins

TIP49b (reptin) is an essential eukaryotic AAA+ ATPase involved in a variety of cellular processes, such as chromatin remodeling during double-strand break repair, transcriptional regulation, control of cell proliferation and small nucleolar RNA biogenesis. How it acts at the molecular level remains largely unknown. In the present study, we show that both human TIP49b and its yeast orthologue, Rvb2p, cooperatively bind single-stranded DNA as monomers. Binding stimulates a slow ATPase activity and supports a 3'- to 5' DNA unwinding activity that requires a 3'-protruding tail >= 30 nucleotides. The data obtained indicate that DNA unwinding of 3'- to 5' junctions is also constrained by the length of flanking duplex DNA. By contrast, TIP49b hexamers were found to be inactive for ATP hydrolysis and DNA unwinding, suggesting that, in cells, protein factors that remain unknown might be required to recycle these into an active form.