Comparative cryo-EM structures of CGMMV and ToMMV reveal divergent coat protein packing and electrostatics indicating Tobamovirus host specificity

Tobamoviruses are a group of economically important plant viruses exhibiting extreme stability and persistence. Despite high morphological similarity, they show extensive host-family specificity, cause a range of symptoms, and cause significant global yield loss in fruit and vegetable plants. Composed of a rod-shaped helical assembly of identical coat proteins encasing a single-stranded RNA, Tobamoviruses have long served as fundamental models for understanding helical assemblies. Here we report the cryo-EM structures of two viruses from this genus, CGMMV and ToMMV, at 2.86 Å and 2.71 Å resolution, respectively. The structures reveal divergent packing strategies within the virion assembly. While ToMMV is genetically similar to the canonical Tobamovirus TMV and displays a structure closely resembling TMV, CGMMV employs a more hydrophobic packing strategy, particularly in the solvent-exposed inner loop region of the coat protein. Comparative biophysical and molecular dynamics analyses from the reconstructed structures reveal more stable and compact protein packing for CGMMV at both the subunit and assembly levels. Furthermore, this study reports a newly identified electrostatic patch existing at the lower alpha-helix region of the coat protein that serves as an additional RNA-stabilizing region, with host family-specific amino acid substitutions among Tobamoviruses. Our study elucidates the structural mechanics and evolutionary strategies driving virion stability and host diversity while maintaining a conserved helical assembly. These findings reveal how subtle electrostatic tuning and divergent packing strategies drive viral stability and host adaptation within a strictly conserved quaternary scaffold.