Structure of the human RECQ1 helicase reveals a putative strand-separation pin

Abstract

RecQ-like helicases, which include 5 members in the human genome, are important in maintaining genome integrity. We present a crystal structure of a truncated form of the human RECQ1 protein with Mg-ADP. The truncated protein is active in DNA fork unwinding but lacks other activities of the full-length enzyme: disruption of Holliday junctions and DNA strand annealing. The structure of human RECQ1 resembles that of Escherichia coli RecQ, with some important differences. All structural domains are conserved, including the 2 RecA-like domains and the RecQ-specific zinc-binding and winged-helix (WH) domains. However, the WH domain is positioned at a different orientation from that of the E. coli enzyme. We identify a prominent ␤-hairpin of the WH domain as essential for DNA strand separation, which may be analogous to DNA strand-separation features of other DNA helicases. This hairpin is significantly shorter in the E. coli enzyme and is not required for its helicase activity, suggesting that there are significant differences between the modes of action of RecQ family members.DNA helicase ͉ DNA repair ͉ Holliday junction ͉ structural genomics ͉ winged helix T he RecQ helicases are a family of DNA-unwinding enzymes conserved from prokaryotes to mammals that play a key role in the maintenance of genome stability. The RecQ helicase family has 5 representatives in the human genome (1-3): RECQ1 (also known as RECQL or RECQL1), BLM, WRN, RECQ4, and RECQ5. Although these 5 enzymes are similar in their catalytic core, they probably have distinct functions, as indicated by the genetic disorders associated to mutations in the genes of BLM, WRN, and RECQ4. In particular, mutations in the gene encoding for BLM (4) are associated with the Bloom's syndrome (BS), which is manifested as an increased incidence of a wide spectrum of cancers. Werner's syndrome (WS), which is linked to mutations in the WRN (5) gene, involves many signs of premature aging, as well as a predisposition to a more limited spectrum of cancers. Mutations in the gene of RECQ4 are the cause of more varied genetic disease phenotypes, including Rothmund-Thomson (RTS) (6, 7), RAPADILINO (8), and Baller-Gerold (9) syndromes. No disease phenotypes have been associated with mutations in the genes of the other 2 family members, RECQ1 and RECQ5 yet, although they may be responsible for additional cancer predisposition disorders that are distinct from RTS, BS, and WS. In this regard, interesting candidates are patients with a phenotype similar to that of RTS individuals who do not carry any mutations in the RECQ4 gene (7). A possible role of RECQ1 in genome maintenance is suggested by several observations (reviewed in ref 10). Biochemical purification from human embryonic kidney cells recovered RECQ1 as the major Holliday junction (HJ) branch migration activity (11). Knockout of the RECQ1 gene in mice (12) or suppression of its expression in HeLa cells (11) resulted in cellular phenotypes that include chromosomal instability, increased sister chromatid exchange, and hei...

Structural mechanisms of DNA binding and unwinding in bacterial RecQ helicases

“…2B) (11,12,22). Loops from the WH domain (residues 432-434 and 441-446) and His489 from the WH β-hairpin contacted the phosphodiester backbone on either side of the DNA major groove ( Fig.…”

“…A third puzzling feature of RecQs is their varied use of a WH domain β-hairpin in DNA unwinding. In eukaryotic RecQ helicases, this β-hairpin projects away from the WH domain, forming a wedge that appears to separate DNA strands (11,12,22). However, the equivalent β-hairpin in EcRecQ is much shorter and is dispensable for helicase activity (10, 11), indicating that the bacterial and human RecQ proteins use different strand-separation mechanisms.…”

“…In eukaryotic RecQ helicases, this β-hairpin projects away from the WH domain, forming a wedge that appears to separate DNA strands (11,12,22). However, the equivalent β-hairpin in EcRecQ is much shorter and is dispensable for helicase activity (10,11), indicating that the bacterial and human RecQ proteins use different strand-separation mechanisms. This finding is even more surprising given that putative wedge elements have been identified in all other SF1 and SF2 DNA helicases of known structure (23).…”

“…1B and Fig. S1) (10)(11)(12). The functional relevance of the open and closed arrangements is unknown.…”

RECQL: a new breast cancer susceptibility gene

“…Sequencing the RECQ1 mRNA from patients with this mutation revealed a 27 nucleotide net addition to the mRNA resulting in deletion of a lysine and addition of 10 amino acids that displaced a conserved b-hairpin, previously shown to be essential for RECQL helicase activity. 6 The RECQL truncating mutation c.634C >T; p. Arg215* that appeared twice in the French population was further screened in 538 patients and 7,136 newborn controls of French-Canadian descent and it was detected in 5 patients and one control, a 50-fold elevated frequency in affected versus unaffected individuals.…”