Characterization of polymorphism displayed by the coat protein mutants of tomato bushy stunt virus

Expression of full-length and N-terminal deletion mutants of the coat protein (CP) of tomato bushy stunt virus (TBSV) using the recombinant baculovirus system resulted in spontaneously assembled virus-like particles (VLPs). Deletion of the majority of the R-domain sequence of the CP, residues 1-52 (CP-NΔ52) and 1-62 (CP-NΔ62), produced capsids similar to wild-type VLPs. Interestingly, the CP-NΔ62 mutant that retains the last 3 residues of R-domain is capable of forming both the T = 1 and T = 3 particles. However, between the two types of VLPs, formation of the T = 1 capsids appears to be preferred. Another mutant, CP-NΔ72, in which R-domain (residues 1-65) was completely removed but contains most of the β-annulus and extended arm (βA) regions exclusively formed T = 1 particles. These results suggest that as few as 3 residues (63-65) of the R-domain, which includes 2 basic amino acids together with the arm (βA) and β-annulus regions, may be sufficient for the formation of T = 3 particles. However, anywhere between 4 to 13 residues of the R-domain may be required for proper positioning of βA and β-annulus structural elements of the C-type subunits to facilitate an error free assembly of T = 3 capsids.