Structure and assembly of Sesbania mosaic virus

Abstract

Sesbania mosaic virus (SeMV) is a ss-RNA (4149 nt) plant sobemovirus isolated from farmer’s field around Tirupathi, Andhra Pradesh. The viral capsid (30 nm diameter) consists of 180 copies of protein subunits (MW 29 kDa) organized with icosahedral symmetry. In order to understand the mechanism of assembly of SeMV, a large number of deletion and substitution mutants of the coat protein (CP) were constructed. Recombinant SeMV CP (rCP) as well as the N-terminal rCP deletion mutant ΔN22 were found to assemble in E. coli into virus-like particles (VLPs). ΔN36 and ΔN65 mostly formed smaller particles consisting of 60 protein subunits. Although particle assembly was not affected due to the substitution of aspartates (D146 and D149) that coordinate calcium ions by asparagines, the stability of the resulting capsids was drastically reduced. Deletion of residues forming a characteristic β-annulus at the icosahedral 3-folds did not affect the assembly of VLPs. Mutation of a single tryptophan, which occurs near the icosahedral fivefold axis to glutamate or lysine, resulted in the disruption of the capsid leading to soluble dimers that resembled the quasi-dimer structure of the native virus. Replacement of positively charged residues in the amino terminal segment of CP resulted in the formation of empty shells. Based on these observations, a plausible mechanism of assembly is proposed.