The role of inter-subunit ionic interactions in the assembly of Physalis mottle tymovirus

Physalis mottle tymovirus (PhMV) is a small spherical plant virus with its RNA genome encapsidated in a protein shell made of 180 identical coat protein (CP) subunits. The amino acid residues involved in two interfacial salt bridges, Asp-83/Arg-159 and Arg-68/Asp-150 and Lys-153, were targeted for mutagenesis with a view to delineate the role of interfacial ionic interactions in the subunit folding and assembly of the virus. R159A and D83A-R159A recombinant CP (rCP) mutants formed stable T =3 capsids, indicating that the D83-R159 interfacial salt bridge is dispensable for the folding and assembly of PhMV. However, D150A and R68Q-D150A mutant rCPs were present in the insoluble fraction, suggesting that the R68-D150 interfacial salt bridge is crucial for subunit folding and assembly. Similarly, K153Q, D83A-K153Q, and H69A-K153Q mutant rCPs were present in the insoluble fraction. Interestingly, the R68Q-D150A, D83A-K153Q, and H69A-K153Q double mutant rCPs could be refolded into partially folded soluble heterogeneous aggregates of 14–16 S. The results further confirm our earlier observation that subunit folding and assembly are concerted events in PhMV