Replication protein A physically interacts with the Bloom's syndrome protein and stimulates its helicase activity

Bloom's syndrome is a rare autosomal recessive disorder characterized by ge nomic instability and predisposition to cancer. BLM, the gene defective in Bloom's syndrome, encodes a 159-kDa protein possessing DNA-stimulated ATPas e and ATP-dependent DNA helicase activities. We have examined mechanistic a spects of the catalytic functions of purified recombinant BLM protein, Thro ugh analyzing the effects of different lengths of DNA cofactor on ATPase ac tivity, we provide evidence to suggest that BLM translocates along single-s tranded DNA in a processive manner. The helicase reaction catalyzed by BLM protein was examined as a function of duplex DNA length. We show that BLM: catalyzes unwinding of short DNA duplexes (less than or equal to 71 base pa irs (bp)) but is severely compromised on longer DNA duplexes (greater than or equal to 259-bp), The presence of the human single-stranded DNA-binding protein (human replication protein A (hRPA)) stimulates the BLM unwinding r eaction on the 259-bp partial duplex DNA substrate. Heterologous single-str anded DNA-binding proteins fail to stimulate similarly the helicase activit y of BLM protein. This is the first demonstration of a functional interacti on between BLM and another protein. Consistent with a functional interactio n between hRPA and the BLM helicase, we demonstrate a direct physical inter action between the two proteins mediated by the 70-kDa subunit of RPA. The interactions between BLM and hRPA suggest that the two proteins function to gether in vivo to unwind DNA duplexes during replication, recombination, or repair.