Octahedral particles of bacteriophage MS2 coat protein mutant

 

Pavel Plevka,1 * Kaspars Tars,2 Lars Liljas1

 

1 Department of Cell and Molecular Biology, Uppsala University, Box 596, SE-751 24 Uppsala, Sweden

2 Latvian Biomedical Research and Study Centre, Ratsupites 1, LV 1067, Riga, Latvia

 

A covalent dimer of the bacteriophage MS2 coat protein was created by genetic fusion of two copies of the gene while removing the stop codon of the first gene. The dimer was crystallized in the cubic F432 space group. The organization of the asymmetric unit together with the F432 symmetry results in an arrangement of subunits that corresponds to T=3 octahedral particles. To form an octahedral particle with closed surface, the dimer subunits interact at sharper angles than in the icosahedral arrangement. The main differences in the subunit packing between the octahedral and icosahedral arrangements are located close to the four-fold and five-fold symmetry axes where different sets of loops mediate the contacts. The octahedral particles are probably artefacts created by the particular crystal packing. We have also determined 4.7 Ǻ resolution X-ray electron density of icosahedral particle assembled from covalent coat protein dimers. At the available resolution the structure corresponds to the wild type MS2 virion except for the intersubunit linker regions. The covalent dimer orientation is random with respect to both icosahedral twofold and quasi-twofold symmetry axes. The inter-subunit contacts around icosahedral three and fivefold symmetry axes are mediated by so called FG loops. The MS2 FG loop was shown to have a function in particle assembly and stability. We have created two covalent dimer mutants, one with substitution and the other with 50 residue insertion, in only one of the two FG loops. The mutations if present in a single subunit would obviate its ability to form capsids. The covalent dimer FG loop mutants were able to form virus-like particles. One FG loop in the covalent dimer was sufficient for particle assembly, but the mutant  particles had lowered temperature stability. The formation of virus-like particles by the FG-loop insertion mutant shows possibility for the use of the covalent dimer variants as epitope carriers.