Structure of the turnip yellows virus particles. How is the readthrough domain exposed on the capsid surface?

The polerovirus turnip yellows virus (TuYV) (formerly beet western yellows virus, BWYV) is a member of the genus Polerovirus in the family Solemoviridae . It is transmitted from plant to plant by aphids according to a circulative and non-propagative mode. TuYV has a wide host range among herbaceous plants and infects important crops such as oilseed rape. Members of the Solemoviridae family are strictly restricted to the three cell types constituting the phloem; the nucleated phloem parenchyma cells and companion cells, where the virus replicates, and the sieve elements, which convey the virus to sites distant from the inoculation point. TuYV consists of a single-stranded positive sense RNA genome of approximately 6 kb encapsidated into an icosahedral capsid of approximately 30 nm in diameter. The capsid is composed of two types of subunits: the major capsid protein (CP, ∼23 kDa) and a minor component, denoted RTP* (∼54 kDa), which arises from the C-terminal cleavage of a readthrough gene product (RTP, 74 kDa). The minor capsid component contains the CP sequence at its N-terminus fused with a structured domain (denoted ^N RTD) which has been shown to be a key determinant of virus transmission and necessary for crossing aphid gut epithelial cells during viral uptake. Though both the CP and RTP* structural proteins are involved in virus movement and aphid transmission, how RTP* is incorporated in the viral particles is poorly understood. We present here the structural characterisation of the TuYV wild-type virions (viral capsids containing both CP and RPT*) and a mutant whose capsids contain the CP only. The comparison of the cryo-EM maps of the wild-type and the mutated virus (resolved at 4.1 and 3.5 Å respectively) reveals that the RTP* does not impair the structural organization of the capsid shell. Furthermore, electron cryo-microscopy and immunogold labelling observations of both viruses indicate that the ^N RTDs do not adopt well-defined positions relative to the capsid surface, and that only four to six ^N RTDs or ^N RTD dimers are incorporated into the viral particles, suggesting a random insertion of the RTP* into the TuYV capsid.