Mutations Altering the Interplay between GkDnaC Helicase and DNA Reveal an Insight into Helicase Unwinding

Abstract

Replicative helicases are essential molecular machines that utilize energy derived from NTP hydrolysis to move along nucleic acids and to unwind double-stranded DNA (dsDNA). Our earlier crystal structure of the hexameric helicase from Geobacillus kaustophilus HTA426 (GkDnaC) in complex with single-stranded DNA (ssDNA) suggested several key residues responsible for DNA binding that likely play a role in DNA translocation during the unwinding process. Here, we demonstrated that the unwinding activities of mutants with substitutions at these key residues in GkDnaC are 2–4-fold higher than that of wild-type protein. We also observed the faster unwinding velocities in these mutants using single-molecule experiments. A partial loss in the interaction of helicase with ssDNA leads to an enhancement in helicase efficiency, while their ATPase activities remain unchanged. In strong contrast, adding accessory proteins (DnaG or DnaI) to GkDnaC helicase alters the ATPase, unwinding efficiency and the unwinding velocity of the helicase. It suggests that the unwinding velocity of helicase could be modulated by two different pathways, the efficiency of ATP hydrolysis or protein-DNA interaction.

Mechanisms for initiating cellular DNA replication

“…Adjoining NTDs in the DnaB hexamer self-assemble into a trimer-of-dimers configuration (127)(128)(129)133), creating a collar that binds ssDNA (127). The CTD tier also interacts with nucleic acid substrates but serves as the site for DNA translocation (134)(135)(136)(137). During replication, DnaB encircles the lagging template DNA strand (138), moving 5′→3′ with the ATPase domains oriented toward the duplex side of the fork (138)(139)(140)(141).…”