Dynamic allostery controls coat protein conformer switching during MS2 phage assembly

Previously, an RNA stem-loop (TR) encompassing 19 nt of the genome of bacteriophage MS2 was shown to act as an allosteric effector of conformational switching in the coat protein during in vitro capsid assembly. TR RNA binding to symmetric coat protein dimers results in conformational changes, principally at the FG-loop connecting the F and G beta-strands in each subunit, yielding an asymmetric structure. The FG-loops define the quasi-equivalent conformers of the coat protein subunit (A, B, and C) in the T=3 capsid. Efficient assembly of this capsid in vitro requires that both symmetrical and asymmetrical forms of the coat protein dimer be present in solution, implying that they closely resemble the quasi-equivalent dimers (A/B and C/C) seen in the final capsid. Experiments show that assembly can be triggered by a number of RNA stem-loops unrelated to TR in sequence and detailed secondary structure, suggesting that there is little sequence specificity to the allosteric effect. Since the stem-loop binding site on the coat protein dimer is distal to the FG-loops the mechanism of this switching effect needs to be investigated. We have analyzed the vibrational modes of both TR-bound and RNA-free coat protein dimers using an all-atom normal-mode analysis. The results suggest that asymmetric contacts between the A-duplex RNA phosphodiester backbone and the EF-loop in one coat protein subunit result in the FG-loop of that subunit becoming more dynamic, whilst the equivalent loop on the other monomer decreases its mobility. The increased dynamic behaviour occurs in the FG-loop of the subunit required to undergo the largest conformational change when adopting the quasi-equivalent B conformation. The free energy barrier on the pathway to form this new structure would consequently be reduced compared to the unbound subunit. Our results also imply that the allosteric effect should be independent of the base sequence of the bound stem-loop, as observed experimentally. As a test of this model, we also examined the vibrational modes of a known assembly mutant, W82R, which cannot assemble beyond dimer. This mutation leads to an increased mobility of the DE-loop rather than the FG-loop after TR binding, consistent with the non-assembling phenotype of this mutant protein.