The Bunyamwera orthobunyavirus Gc glycoprotein head and stalk drives an infectious virion assembly pathway specific for the insect host.
The Orthobunyavirus genus of arthropod-borne segmented RNA viruses comprises important pathogens including the human-infecting Oropouche virus and ruminant-infecting Schmallenberg virus (SBV). The prototypical Bunyamwera orthobunyavirus (BUNV) possesses envelope-embedded glycoprotein Gn-Gc tripodal spikes, of which the ectodomains mediate virus entry, while endodomains interact with nucleoprotein (NP) enwrapped genome segments driving virion assembly. Interestingly, BUNV Gc head/stalk domains are redundant for virus growth in mammalian cells, consistent with isolations of SBV from ruminants bearing head/stalk deletions. However, these domains appear strictly maintained in orthobunyavirus isolations from arthropods in nature. To investigate the molecular mechanism that underlines this discrepancy, we compared the multiplication characteristics of wildtype BUNV (BUNV-WT) with a Gc head/stalk deleted BUNV (BUNV-∆7). In mammalian cells BUNV-WT and BUNV-∆7 grew to equivalent titres, whereas in insect cells BUNV-∆7 titres were 1000-fold lower and strikingly produced no virions following blood meal infection of Aedes mosquitoes. To understand this insect-specific restriction in virion production, we showed the intracellular abundance of BUNV-WT and BUNV-∆7 Gc and NP components were equivalent, suggesting the deletion impacted post-translational stages of the infection cycle. To explore this, we investigated Gc and ∆7-Gc interactions during BUNV-WT and BUNV-∆7 infections of both insect and mammalian cells by co-immunoprecipitation and multiplex mass spectrometry, revealing ∆7-Gc exhibited markedly reduced NP interactions in insect cells, potentially indicating reduced segment interactions during assembly. We hypothesize that the Gc head/stalk performs an insect cell-specific role in segment recruitment during virion formation, and that maintenance in nature of full-length Gc is due to this essential role in the insect host.