The Role of Arginine-rich Motif and β-Annulus in the Assembly and Stability of Sesbania Mosaic Virus Capsids

Sesbania mosaic virus (SeMV) capsids are stabilized by protein–protein, protein–RNA and calcium-mediated protein–protein interactions. The N-terminal random domain of SeMV coat protein (CP) controls RNA encapsidation and size of the capsids and has two important motifs, the arginine-rich motif (ARM) and the β-annulus structure. Here, mutational analysis of the arginine residues present in the ARM to glutamic acid was carried out. Mutation of all the arginine residues in the ARM almost completely abolished RNA encapsidation, although the assembly of T=3 capsids was not affected. A minimum of three arginine residues was found to be essential for RNA encapsidation. The mutant capsids devoid of RNA were less stable to thermal denaturation when compared to wild-type capsids. The results suggest that capsid assembly is entirely mediated by CP-dependent protein–protein inter-subunit interactions and encapsidation of genomic RNA enhances the stability of the capsids. Because of the unique structural ordering of β-annulus segment at the icosahedral 3-folds, it has been suggested as the switch that determines the pentameric and hexameric clustering of CP subunits essential for T=3 capsid assembly. Surprisingly, mutation of a conserved proline within the segment that forms the β-annulus to alanine, or deletion of residues 48–53 involved in hydrogen bonding interactions with residues 54–58 of the 3-fold related subunit or deletion of all the residues (48–59) involved in the formation of β-annulus did not affect capsid assembly. These results suggest that the switch for assembly into T=3 capsids is not the β-annulus. The ordered β-annulus observed in the structures of many viruses could be a consequence of assembly to optimize intersubunit interactions.