The Molecular Aspects Of RecQ Helicase Family

In the cells, the unwinding of double-stranded polynucleotides is catalyzed by helicases that exist in all kingdoms of life from virus to human. RecQ family helicases play essential roles in nucleic acid metabolism by facilitating cellular processes including genome replication, DNA repair, recombination, transcription and telomere maintenance. In human, five RecQ family members named RECQ1, BLM, WRN, RECQ4 and RECQ5, have been identified. Defects in BLM, WRN and RECQ4 will give rise to autosomal recessive disorders and predisposition to cancer. In addition to the highly conserved helicase core domain containing seven helicase motifs, most RecQ family helicases have a unique RecQ C-terminal domain (RecQ-Ct) and the Helicase RNase D conserved domain (HRDC). In the present studies, we focus on the intra-functional mechanisms of some important members of RecQ family helicases.Firstly, we have chosen two natural isoforms of human RECQ5 helicase as models to study the functional modulation of the helicase core domain by the zinc-binding domain. Here we show that a truncated variant of the human RECQ5βhelicase comprised of the conserved helicase domain only, a splice variant named RECQ5α, possesses neither ATPase nor DNA unwinding activities, but surprisingly displays a strong strand annealing activity. Quantitative measurements indicate that the regulatory role of the zinc-binding motif of RECQ5βis achieved by enhancing the DNA binding affinity of the helicase. More important, the zinc-binding motif of RECQ5βis found to act as a molecular switch that suppresses the strand-annealing activity of the helicase domain and triggers DNA-unwinding activity of the enzyme through enhancing DNA binding.Subsequently, we analyzed the biochemical properties of two isoforms of Bacillus subtilis RecQ helicases: SubL and SubS. Between them, SubS naturally lacks the HRDC domain. Our studies demonstrate that the HRDC domain is crucial in Bacillus subtilis RecQ helicases in unwinding of DNA replication/repair intermediates such as Holliday junction and kappa junction. The enzyme with HRDC domain shows stronger ATPase activity and DNA unwinding and annealing activities than the other one. These results allow us to speculate on the importance of HRDC domain in the basic activities of RecQ family helicases.In the last part, we investigated the existence and role of the arginine finger in the Bloom syndrome protein (BLM) in ATP hydrolysis and energy coupling. Our studies demonstrate that R982 is the residue located near theγ-phosphate of ATP which functions as a BLM arginine finger. Our finding further indicates that the arginine finger interacts with other conserved motifs aroundγ-phosphate of the nucleotide to carry out the functions as a complex network.