Structure of phage fr capsids with a deletion in the FG loop: implications for viral assembly

The loop between beta-strands F and G in the coat protein of small RNA bacteriophages forms the interactions at the fivefold and threefold (quasi-sixfold) icosahedral axes. In many cases, mutations in this region renders the coat protein unable to form capsids. This FG loop has therefore been suggested to be of major importance for the virus assembly process by guiding the assembly and helping to define the correct curvature of the virus shell. We have determined the crystal structure of a phage fr capsid where the coat protein has a four-residue deletion in the FG loop. This mutant retains the ability to form virus capsids of normal size but has a significantly lower temperature stability than the wild type. The structure reveals that the mutated loops are flexible and too short to interact with each other. This seems incompatible with a role of the FG loop in the regulation of capsid size.