The cryo-electron microscopy structure of Broad Bean Stain Virus suggests a common capsid assembly mechanism among comoviruses

Recent advances in the structure and assembly of non-enveloped spherical viruses

Non-enveloped spherical viruses (NSVs) are characterized by their highly symmetrical capsids that serve to protect and encapsulate the genomes. The stability and functionality of the capsids determine their ability for survival and proliferation in harsh environments. Over four decades of structural studies using X-ray crystallography and NMR have provided static, high-resolution snapshots of several viruses. Recently, advances in cryo-electron microscopy, together with AI-based structure predictions and traditional methods, have aided in elucidating not only the structural details of complex NSVs but also the mechanistic processes underlying their assembly. The knowledge thus generated has been instrumental in critical understanding of the conformational changes and interactions associated with the coat proteins, the genome, and the auxiliary factors that regulate the capsid dynamics. This review seeks to summarize current literature regarding the structure and assembly of the NSVs and discusses how the data has facilitated a deeper understanding of their biology and phylogeny.

Complete genome sequences of two different broad bean stain virus isolates

Here, the complete genome sequences of two broad bean stain virus (BBSV; genus Comovirus) strains - Tunisia (RNA1, 5,965 nt; RNA2, 3,467 nt) and Syria (RNA1, 5,976 nt; RNA2, 3,449 nt) - were determined using next-generation sequencing (NGS) and rapid amplification of cDNA ends (RACE). These strains showed 79.2% and 71.6% nucleotide sequence identity to each other in RNA1 and RNA2, respectively. The predicted amino acid sequence identity was 95.2% in the proteinase-polymerase and 93.9% in the coat protein, indicating that these strains belong to the same species in the genus Comovirus. Phylogenetic analysis showed that both strains are closely related to red clover mottle virus (genus Comovirus). This is the first report of the complete genome sequences of two BBSV strains.

Apple latent spherical virus structure with stable capsid frame supports quasi-stable protrusions expediting genome release

Picorna-like plant viruses are non-enveloped RNA spherical viruses of ~30 nm. Part of the survival of these viruses depends on their capsid being stable enough to harbour the viral genome and yet malleable enough to allow its release. However, molecular mechanisms remain obscure. Here, we report a structure of a picorna-like plant virus, apple latent spherical virus, at 2.87 Å resolution by single-particle cryo-electron microscopy (cryo-EM) with a cold-field emission beam. The cryo-EM map reveals a unique structure composed of three capsid proteins Vp25, Vp20, and Vp24. Strikingly Vp25 has a long N-terminal extension, which substantially stabilises the capsid frame of Vp25 and Vp20 subunits. Cryo-EM images also resolve RNA genome leaking from a pentameric protrusion of Vp24 subunits. The structures and observations suggest that genome release occurs through occasional opening of the Vp24 subunits, possibly suppressed to a low frequency by the rigid frame of the other subunits.