Abstract
Bloom's syndrome helicase (BLM) is a member of the RecQ family of DNA helicases, which play key roles in the maintenance of genome integrity in all organism groups. We describe crystal structures of the BLM helicase domain in complex with DNA and with an antibody fragment, as well as SAXS and domain association studies in solution. We show an unexpected nucleotide-dependent interaction of the core helicase domain with the conserved, poorly characterized HRDC domain. The BLM–DNA complex shows an unusual base-flipping mechanism with unique positioning of the DNA duplex relative to the helicase core domains. Comparison with other crystal structures of RecQ helicases permits the definition of structural transitions underlying ATP-driven helicase action, and the identification of a nucleotide-regulated tunnel that may play a role in interactions with complex DNA substrates.
Highlights
Bloom’s syndrome (BS) is an extremely rare autosomal recessive disorder that is characterized by shortness of stature, a distinctive skin rash and the predisposition to the development of a wide spectrum of cancers at an early age [1]
Data were processed with XDS [38], and the structure was solved by molecular replacement using the program PHASER [39] and the Bloom’s syndrome helicase (BLM) nanobody complex as a search model, with the DNA being modelled manually into the electron density maps after several rounds of refinement using the program COOT [40]
Many features of the interaction of BLM with adenosine diphosphate (ADP) and with the 3 signle-stranded DNA (ssDNA) overhang are similar to those observed in other superfamily 2 (SF2) helicases, involving residues of the conserved helicase motifs
Summary
Bloom’s syndrome (BS) is an extremely rare autosomal recessive disorder that is characterized by shortness of stature, a distinctive skin rash and the predisposition to the development of a wide spectrum of cancers at an early age [1]. Bloom’s syndrome helicase (BLM), like all RecQ-family helicases, acts as a 3 to 5 helicase [7] on a wide variety of DNA substrates including forked duplexes, G quadruplexes, 4-way junctions [8] and displacement loops (D loops) [9] It forms a multi protein complex with human topoisomerase III␣ [10], RMI1 [11] and RMI2 [12], termed the ‘dissolvasome’, which can promote the convergent branch migration [13] and decatenation of double Holliday junction intermediates formed during homologous recombination. This dissolution reaction prevents the exchange of genetic material flanking two homologous sequences engaged in homologous recombination [14,15]
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