ABSTRACTThe flagellated protozoanTrichomonas vaginalisis an obligate human genitourinary parasite and the most frequent cause of sexually transmitted disease worldwide. Most clinical isolates ofT. vaginalisare persistently infected with one or more double-stranded RNA (dsRNA) viruses from the genusTrichomonasvirus, familyTotiviridae, which appear to influence not only protozoan biology but also human disease. Here we describe the three-dimensional structure ofTrichomonas vaginalisvirus 1 (TVV1) virions, as determined by electron cryomicroscopy and icosahedral image reconstruction. The structure reveals aT= 1 capsid comprising 120 subunits, 60 in each of two nonequivalent positions, designated A and B, as previously observed for fungalTotiviridaefamily members. The putative protomer is identified as an asymmetric AB dimer consistent with either decamer or tetramer assembly intermediates. The capsid surface is notable for raised plateaus around the icosahedral 5-fold axes, with canyons connecting the 2- and 3-fold axes. Capsid-spanning channels at the 5-fold axes are unusually wide and may facilitate release of the viral genome, promoting dsRNA-dependent immunoinflammatory responses, as recently shown upon the exposure of human cervicovaginal epithelial cells to either TVV-infectedT. vaginalisor purified TVV1 virions. Despite extensive sequence divergence, conservative features of the capsid reveal a helix-rich fold probably derived from an ancestor shared with fungalTotiviridaefamily members. Also notable are mass spectrometry results assessing the virion proteins as a complement to structure determination, which suggest that translation of the TVV1 RNA-dependent RNA polymerase in fusion with its capsid protein involves −2, and not +1, ribosomal frameshifting, an uncommonly found mechanism to date.IMPORTANCETrichomonas vaginaliscauses ~250 million new cases of sexually transmitted disease each year worldwide and is associated with serious complications, including premature birth and increased transmission of other pathogens, including HIV. It is an extracellular parasite that, in turn, commonly hosts infections with double-stranded RNA (dsRNA) viruses, trichomonasviruses, which appear to exacerbate disease through signaling of immunoinflammatory responses by human epithelial cells. Here we report the first three-dimensional structure of a trichomonasvirus, which is also the first such structure of any protozoan dsRNA virus; show that it has unusually wide channels at the capsid vertices, with potential for releasing the viral genome and promoting dsRNA-dependent responses by human cells; and provide evidence that it uses −2 ribosomal frameshifting, an uncommon mechanism, to translate its RNA polymerase in fusion with its capsid protein. These findings provide both mechanistic and translational insights concerning the role of trichomonasviruses in aggravating disease attributable toT. vaginalis.
Purified Recombinant Bluetongue Virus VP1 Exhibits RNA Replicase Activity
