Trichomonas vaginalis, the causative pathogen for the most common nonviral
sexually transmitted infection worldwide, is itself frequently infected with one or more of the four types of small
double-stranded RNA (dsRNA) Trichomonas vaginalis viruses (TVV1 to 4, genus Trichomonasvirus, family Totiviridae). Each TVV encloses a nonsegmented genome within a single-layered capsid and replicates entirely intracellularly, like many dsRNA viruses, and unlike those in the Reoviridae family. Here, we have determined the structure of TVV2 by cryo-electron microscopy (cryoEM) at 3.6 Å resolution and derived an atomic model of its capsid. TVV2 has an icosahedral, T = 2*, capsid comprised of 60 copies of the icosahedral asymmetric unit (a dimer of the two capsid shell
protein [CSP] conformers, CSP-A and CSP-B), typical of icosahedral dsRNA virus capsids. However, unlike the robust CSP-interlocking interactions such as the use of auxiliary "clamping"
proteins among Reoviridae, only lateral CSP interactions are observed in TVV2, consistent with an assembly strategy optimized for TVVs' intracellular-only replication cycles within their protozoan host. The atomic model reveals both a mostly negatively charged capsid interior, which is conducive to movement of the loosely packed genome, and channels at the 5-fold vertices, which we suggest as routes of
mRNA release during transcription. Structural comparison of TVV2 to the Saccharomyces cerevisiae L-A virus reveals a conserved helix-rich fold within the CSP and putative
guanylyltransferase domain along the capsid exterior, suggesting conserved
mRNA maintenance strategies among Totiviridae This first atomic structure of a TVV provides a framework to guide future biochemical investigations into the interplay between Trichomonas vaginalis and its viruses.IMPORTANCETrichomonas vaginalis viruses (TVVs) are
double-stranded RNA (dsRNA) viruses that cohabitate in Trichomonas vaginalis, the causative pathogen of
trichomoniasis, the most common nonviral
sexually transmitted disease worldwide. Featuring an unsegmented dsRNA genome encoding a single capsid shell
protein (CSP), TVVs contrast with multisegmented dsRNA viruses, such as the
diarrhea-causing rotavirus, whose larger genome is split into 10 dsRNA segments encoding 5 unique
capsid proteins. To determine how TVVs incorporate the requisite functionalities for viral replication into their limited
proteome, we derived the atomic model of TVV2, a first for TVVs. Our results reveal the intersubunit interactions driving CSP association for capsid assembly and the properties that govern organization and maintenance of the viral genome. Structural comparison between TVV2 capsids and those of distantly related dsRNA viruses indicates conserved strategies of nascent
RNA release and a putative viral
guanylyltransferase domain implicated in the cytoplasmic maintenance of viral messenger and genomic
RNA.