On translocation mechanism of ring-shaped helicase along single-stranded DNA

Non-tight and tight chemomechanical couplings of biomolecular motors under hindering loads

Biomolecular motors make use of free energy released from chemical reaction (typically ATP hydrolysis) to perform mechanical motion or work. An important issue is whether a molecular motor exhibits tight or non-tight chemomechanical (CM) coupling. The tight CM coupling refers to that each ATPase activity is coupled with a mechanical step, while the non-tight CM coupling refers to that an ATPase activity is not necessarily coupled with a mechanical step. Here, we take kinesin, monomeric DNA helicase, ring-shaped hexameric DNA helicase and ribosome as examples to study this issue. Our studies indicate that some motors such as kinesin, monomeric helicase and ribosome exhibit non-tight CM coupling under hindering forces, while others such as the ring-shaped hexameric helicase exhibit tight or nearly tight CM coupling under any force. For the former, the reduction of the velocity caused by the hindering force arises mainly from the reduction of the CM coupling efficiency, while the ATPase rate is independent or nearly independent of the force. For the latter, the reduction of the velocity caused by the hindering force arises mainly from the reduction of the ATPase rate, while the CM coupling efficiency is independent or nearly independent of the force.

A unified model of nucleic acid unwinding by the ribosome and the hexameric and monomeric DNA helicases

DNA helicases are enzymes that use the chemical energy to separate DNA duplex into their single-stranded forms. The ribosome, which catalyzes the translation of messenger RNAs (mRNAs) into proteins, can also unwind mRNA duplex. According to their structures, the DNA helicases can fall broadly into hexameric and monomeric forms. A puzzling issue for the monomeric helicases is that although they have similar structures, in vitro biochemical data showed convincingly that in the monomeric forms some have very weak DNA unwinding activities, some have relatively high unwinding activities while others have high unwinding activities. However, in the dimeric or oligomeric forms all of them have high unwinding activities. In addition, in the monomeric forms all of them can translocate efficiently along the single-stranded DNA (ssDNA). Here, we propose a model of the translocation along the ssDNA and DNA unwinding by the monomeric helicases, providing a consistent explanation of these in vitro experimental data. Moreover, by comparing the present model for the monomeric helicases with the model for the hexameric helicases and that for the ribosome which were proposed before, a unified model of nucleic acid unwinding by the three enzymes is proposed.

Processive and unidirectional translocation of monomeric UvsW helicase on single-stranded DNA

UvsW protein from bacteriophage T4 controls the transition from origin-dependent to origin-independent initiation of replication through the unwinding of R-loops bound to the T4 origins of replication. UvsW has also been implicated through genetic and biochemical experiments to play a role in DNA repair processes such as replication fork regression and Holliday junction branch migration. UvsW is capable of unwinding a wide variety of substrates, many of which contain only duplex DNA without single-stranded regions. Based on this observation, it has been suggested that UvsW is a dsDNA translocase. In this work we examine the ability of UvsW to translocate on ssDNA. Kinetic analysis indicates that the rate of ATP hydrolysis is strongly dependent on the length of the ssDNA lattice, whereas the K(M)-DNA remains relatively constant, demonstrating that UvsW translocates on ssDNA in an ATP-dependent fashion. Experiments using streptavidin blocks or poly dT sequences located at either end of the ssDNA substrate indicate that UvsW translocates in a 3' to 5' direction. Mutant competition and heparin trapping experiments reveal that UvsW is extremely processive during ATP-driven translocation with a half-life on the order of several minutes. Finally, functional assays provide evidence that UvsW is monomeric while translocating on ssDNA. The ability of UvsW to unwind DNA duplexes is likely to be mechanistically linked to its ability to processively translocate on ssDNA in a 3' to 5' unidirectional fashion.

On chemomechanical coupling of the F(1)-ATPase molecular motor

F(1)-ATPase catalyzes ATP hydrolysis to drive the central gamma-shaft rotating inside a hexameric cylinder composed of alternating alpha and beta subunits. Experiments showed that the rotation of gamma-shaft proceeds in steps of 120 degrees and each 120 degrees -rotation is composed of an 80 degrees substep and a 40 degrees substep. Here, based on the previously proposed models, an improved physical model for chemomechanical coupling of F(1)-ATPase is presented, with which the two-substep rotation is well explained. One substep is driven by the power stroke upon ATP binding, while the other one resulted from the passage of gamma-shaft from previous to next adjacent beta subunits via free diffusion. Using the model, the dynamics and kinetics of F(1)-ATPase, such as the rotating time of each substep, the dwell time at each pause and the rotation rate, are analytically studied. The theoretical results obtained with only three adjustable parameters reproduce the available experimental data well.

Model for RuvAB-mediated branch migration of Holliday junctions

During RuvAB-mediated Holliday-junction migration two opposite arms of double-stranded DNA (dsDNA) are driven to translocate unidirectional by two respective ring-like hexameric RuvB proteins. However, how the RuvB protein, powered by ATP hydrolysis, drives unidirectional translocation of dsDNA is not clear. Here a model is presented for this mechanochemical-coupling mechanism. In the model, the unidirectional translocation is resulted from both the ATP hydrolysis-induced rotation (power stroke) of the RuvB subunits and the passage of the strong DNA binding from the previous to next RuvB subunits during the sequential ATPase activities around the ring. Using the model, the relationship between the power-stroke size, the step size of DNA translocation and the ratio of the rotational rate of DNA over that of RuvB relative to RuvA is predicted.