Structure-Based Genetic Analysis of Hel308a in the Hyperthermophilic Archaeon Sulfolobus islandicus

HELQ as a DNA helicase: Its novel role in normal cell function and tumorigenesis (Review)

Helicase POLQ‑like (HELQ or Hel308), is a highly conserved, 3'‑5' superfamily II DNA helicase that contributes to diverse DNA processes, including DNA repair, unwinding, and strand annealing. HELQ deficiency leads to subfertility, due to its critical role in germ cell stability. In addition, the abnormal expression of HELQ has been observed in multiple tumors and a number of molecular pathways, including the nucleotide excision repair, checkpoint kinase 1‑DNA repair protein RAD51 homolog 1 and ATM/ATR pathways, have been shown to be involved in HELQ. In the present review, the structure and characteristics of HELQ, as well as its major functions in DNA processing, were described. Molecular mechanisms involving HELQ in the context of tumorigenesis were also described. It was deduced that HELQ biology warrants investigation, and that its critical roles in the regulation of various DNA processes and participation in tumorigenesis are clinically relevant.

Phosphorylation of the Archaeal Holliday Junction Resolvase Hjc Inhibits Its Catalytic Activity and Facilitates DNA Repair in Sulfolobus islandicus REY15A

Protein phosphorylation is one of the main protein post-translational modifications and regulates DNA repair in eukaryotes. Archaeal genomes encode eukaryotic-like DNA repair proteins and protein kinases (ePKs), and several proteins involved in homologous recombination repair (HRR) including Hjc, a conserved Holliday junction (HJ) resolvase in Archaea, undergo phosphorylation, indicating that phosphorylation plays important roles in HRR. Herein, we performed phosphorylation analysis of Hjc by various ePKs from Sulfolobus islandicus. It was shown that SiRe_0171, SiRe_2030, and SiRe_2056, were able to phosphorylate Hjc in vitro. These ePKs phosphorylated Hjc at different Ser/Thr residues: SiRe_0171 on S34, SiRe_2030 on both S9 and T138, and SiRe_2056 on T138. The HJ cleavage activity of the phosphorylation-mimic mutants was analyzed and the results showed that the cleavage activity of S34E was completely lost and that of S9E had greatly reduced. S. islandicus strain expressing S34E in replacement of the wild type Hjc was resistant to higher doses of DNA damaging agents. Furthermore, SiRe_0171 deletion mutant exhibited higher sensitivity to DNA damaging agents, suggesting that Hjc phosphorylation by SiRe_0171 enhanced the DNA repair capability. Our results revealed that HJ resolvase is regulated by protein phosphorylation, reminiscent of the regulation of eukaryotic HJ resolvases GEN1 and Yen1.

DNA repair in the archaea-an emerging picture

There has long been a fascination in the DNA repair pathways of archaea, for two main reasons. Firstly, many archaea inhabit extreme environments where the rate of physical damage to DNA is accelerated. These archaea might reasonably be expected to have particularly robust or novel DNA repair pathways to cope with this. Secondly, the archaea have long been understood to be a lineage distinct from the bacteria, and to share a close relationship with the eukarya, particularly in their information processing systems. Recent discoveries suggest the eukarya arose from within the archaeal domain, and in particular from lineages related to the TACK superphylum and Lokiarchaea. Thus, archaeal DNA repair proteins and pathways can represent a useful model system. This review focuses on recent advances in our understanding of archaeal DNA repair processes including base excision repair, nucleotide excision repair, mismatch repair and double-strand break repair. These advances are discussed in the context of the emerging picture of the evolution and relationship of the three domains of life.

The archaeal ATPase PINA interacts with the helicase Hjm via its carboxyl terminal KH domain remodeling and processing replication fork and Holliday junction

PINA is a novel ATPase and DNA helicase highly conserved in Archaea, the third domain of life. The PINA from Sulfolobus islandicus (SisPINA) forms a hexameric ring in crystal and solution. The protein is able to promote Holliday junction (HJ) migration and physically and functionally interacts with Hjc, the HJ specific endonuclease. Here, we show that SisPINA has direct physical interaction with Hjm (Hel308a), a helicase presumably targeting replication forks. In vitro biochemical analysis revealed that Hjm, Hjc, and SisPINA are able to coordinate HJ migration and cleavage in a concerted way. Deletion of the carboxyl 13 amino acid residues impaired the interaction between SisPINA and Hjm. Crystal structure analysis showed that the carboxyl 70 amino acid residues fold into a type II KH domain which, in other proteins, functions in binding RNA or ssDNA. The KH domain not only mediates the interactions of PINA with Hjm and Hjc but also regulates the hexameric assembly of PINA. Our results collectively suggest that SisPINA, Hjm and Hjc work together to function in replication fork regression, HJ formation and HJ cleavage.