Molecular motors: The RuvAB complex

During homologous recombination and DNA repair processes, two homologous double-stranded DNA tracts are joined to form intermediate structures called Holliday junctions  as illustrated below:

In bateria such as E. coli, the RuvAB enzyme complex displaces the junction point in a process called branch migration,  using ATP as an energy source. The goal of our studies is to investigate how this protein motor works.  The RuvAB consists of two RuvA tetramers which bind to a junction with high affinity, and two RuvB hexamers, the motors and ATP burners, which bind to opposing arms of the junction flanking the RuvA tetramer. Upon addition of ATP, the junction is moved as depicted below:

We are currently interested in branch migration induced by the RuvAB complex. This latter consists of an A component which binds to a junction with high affinity, and two B hexameric rings which translocate on DNA.

The movie below shows the gradual defocusing of a bead tethered to a surface by a Holliday junction, as a RuvAB complex shortens the two tethering arms.

Remarkably, the single-molecule experiments show that RuvAB-induced branch migration is characterized by abrupt changes in rate and a processivity of thousands of basepairs. A counteracting force of ~25 pN applied to the junction substrate was necessary to stop branch migration, indicating that the motor is strong enough to displace proteins it may find along its way.

“Direct observation of RuvAB-catalyzed branch migration of single Holliday junctions”
R. Amit, O. Gileadi, and J. Stavans, Proc. Natl. Acad. Sci. 101, 11605 (2004).