Electrostatic Interactions at the Dimer Interface Stabilize the E. coli β Sliding Clamp.

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Sliding clamps, like the dimeric Escherichia coli β-clamp, are essential protein rings for processive DNA replication.
• These clamps encircle DNA and function with DNA polymerases, requiring assembly by a clamp loader.
• The β-clamp transitions from a closed conformation in solution to an open one for DNA loading.

Purpose of the Study:

• To investigate factors stabilizing the closed dimer interface of the β-clamp.
• To understand how clamp dynamics contribute to the open conformation required for DNA binding.
• To elucidate the role of electrostatic interactions and conformational fluctuations in β-clamp function.

Main Methods:

• Studied the effects of varying salt concentrations on dimer stability and subunit dissociation kinetics.
• Utilized equilibrium dissociation constant measurements to analyze salt concentration effects.
• Employed time-resolved fluorescence and fluorescence correlation spectroscopy to probe clamp dynamics.
• Performed site-directed mutagenesis of key interface residues (Arg-103, Ser-109, Ile-305).

Main Results:

• Increased salt concentration destabilized the β-clamp dimer and accelerated subunit dissociation.
• The observed salt dependence of the dissociation constant aligns with charge-screening models, highlighting electrostatic contributions.
• Mutation of Arg-103 significantly weakened the dimer interface, unlike mutations of Ser-109 or Ile-305.
• Fluorescence experiments ruled out significant spontaneous fluctuations between open and closed clamp conformations.

Conclusions:

• Electrostatic interactions, particularly involving charged residues like Arg-103, are critical for the stability of the dimeric β-clamp interface.
• The mechanism of clamp opening by clamp loaders does not appear to rely on substantial pre-existing conformational fluctuations of the clamp.
• Understanding these dynamics is crucial for comprehending the regulation of DNA replication fidelity.