Structures of T = 1 and T = 3 Particles of Cucumber Necrosis Virus: Evidence of Internal Scaffolding

Abstract

Cucumber necrosis virus (CNV) is a member of the genus Tombusvirus, 1,2 of which tomato bushy stunt virus (TBSV) is the type member. The capsid protein for this group of viruses is composed of three major domains: the R domain, which interacts with the RNA genome: the S domain, which forms the tight capsid shell: and the protruding P domain, which extends ∼40 Å from the surface. 3 Here, we present the cryo-transmission electron microscopy structures of both the T = 1 and T = 3 capsids to a resolution of ∼12 Å. The T = 3 capsid is essentially identical with that of TBSV, and the T = 1 particles are well described by the A subunit pentons from TBSV. Perhaps most notable is the fact that the T = 3 particles have an articulated internal structure with two major internal shells, while the internal core of the T = 1 particle is essentially disordered. These internal shells of the T = 3 capsid agree extremely well in both dimension and character with published neutron-scattering results. This structure, combined with mutagenesis results in the accompanying article, suggests that the R domain forms an internal icosahedral scaffold that may play a role in T = 3 capsid assembly. In addition, the N-terminal region has been shown to be involved in chloroplast targeting. 4 Therefore, this region apparently has remarkably diverse functions that may be distributed unevenly among the quasi-equivalent A, B, and C subunits.