The RecQ DNA helicases in DNA repair

Abstract

The RecQ helicases are conserved from bacteria to humans and play a critical role in genome stability. In humans, loss of RecQ gene function is associated with cancer predisposition and/or premature aging. Recent experiments have shown that the RecQ helicases function during distinct steps during DNA repair; DNA end resection, displacement-loop (D-loop) processing, branch migration, and resolution of double Holliday junctions (dHJs). RecQ function in these different processing steps has important implications for its role in repair of double-strand breaks (DSBs) that occur during DNA replication and meiosis, as well as at specific genomic loci such as telomeres.

Figure 1

Structural features of RecQ helicases. The RecQ proteins have many structural domains that are conserved from bacteria through humans. The RecQ proteins all have a core helicase domain (red). Most RecQ proteins also contain conserved HRDC (Helicase and RNAse D C-terminal; yellow) and RQC (RecQ C-terminal; turquoise) domains that are thought to mediate interactions with nucleic acid or other proteins respectively. Many RecQ proteins have acidic regions (green) that enable protein-protein interactions while some of the RecQ proteins have nuclear localization sequences (NLS; orange). WRN and FFA-1 protein are unique in that they also contain an exonuclease domain (EXO; purple). The number of amino acids for each protein are indicated on the right.

Figure 2

Conserved interaction between DNA repair and recombination proteins with Sgs1 and BLM. In yeast, Sgs1 physically interacts with many different repair and recombination proteins as indicated such as Top3 and Rmi1. Many of these interactions are evolutionarily conserved and are shown with the BLM protein. BLM also interacts with one of the other RecQ helicases, WRN. Mlh1 has an interaction site in the C-terminal region of both proteins (not shown). The colors for Sgs1 and BLM are the same as in Figure 1.

Figure 3

Model for RecQ function during end resection. After a DSB, the DNA ends are recognized by Mre11-Rad50-Xrs2 (MRX) in yeast or MRE11-RAD50-NBS1 (MRN) in mammals. The DNA ends are partially resected by Mre11/MRE11 nucleases, in collaboration with Sae2/CtIP endonucleases, leaving short 3’ single-stranded DNA tails. These DNA ends can be further resected by utilizing Sgs1/BLM helicases and Dna2/DNA2 nucleases or via a parallel pathway that uses Exo1/EXO1 nucleases. The ssDNA created is coated by RPA.

Figure 4

Model for RecQ function during junction processing and resolution. Junction processing: The Rad52 group of proteins recruits Rad51 and displaces RPA leading to Rad51 filament formation. Rad51 filaments perform homology search and strand invasion leading to D-loop formation followed by branch migration. Rad51 coated DNA can invade using 5’ or 3’ DNA ends; however, only the 3’ invading end is proficient for homologous recombination. The RecQ proteins can unwind the 5’ end and thus abort the unproductive reaction but also the 3’ end after it is extended to promote synthesis dependent strand annealing (not shown). Alternatively, second end capture of the homologous chromosome can lead to double Holliday junction formation (dHJ). The RecQ proteins function by promoting translocation and branch migration of dHJs. Resolution: Dissolution of dHJs utilize the Sgs1-Top3-Rmi1/BLM-TOP3α-RMI1(BLAP75) complex. In the figure the yeast proteins are shown bound to each HJ and if they move toward each other dissolution can occur. However, the precise biochemical reaction is not known. During DNA replication, hemicatenenes are thought to form behind the replication fork (17). In vitromaximal decatenation of this structure is achieved when Rmi1 and Rmi2 are added to the reaction.

Figure 5

DNA structures unwound by one or multiple RecQ helicases. The RecQ proteins unwind a diverse set of DNA structures in vitro such as blunt ended DNA (however, not the preferred substrate), 3’ overhangs, forked substrates (such as those that arise during DNA replication), displacement-loops (D-loops, that arise during strand invasion), 4-way junctions (similar to a Holliday junction, HJ), synthetic dHJs (mimicking a recombination intermediate), or structures capable of forming G-quadruplex DNA (for example, predicted in rDNA and telomere sequences).