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. 2023 Aug 16;15(8):1750. doi: 10.3390/v15081750

Figure 4.

Figure 4

Assembly of virions. Primary transcription occurs immediately after the release of the DLP to the cytoplasm, and capped, non-polyadenylated, positive-sense ssRNAs are extruded from the viral particle, initiating the synthesis of viral proteins (C1). Assembly begins with the specific interaction of proteins VP1 (and possibly VP3) with conserved bases at the 3′ ends of the ssRNA(+), forming pre-core complexes. The 11 different complexes are specifically recruited to the viroplasms (C2), which are biomolecular condensates formed by the liquid–liquid phase separation (LLPS) of viral proteins NSP5 and NSP2, and where the generation of SLP, genome replication, DLP assembly, and secondary transcription occur (C3). TLP assembly occurs within membranous structures surrounding viroplasms, which are derived from COOP-II cisternae (C4), which are sequestered and diverted, along with viral protein VP7, to the vicinity of the viroplasms by the non-structural protein NSP4 (C5). Interaction between DLP-VP6 and NSP4 drives the progressive engulfment of the DLP by NSP4/VP7-containing membranes, resulting in the budding of the DLP–VP4–NSP4–VP7 complex into the lumen of the COPII-derived vesicles, in the form of transient membrane-enveloped particles (eDLP) (C6). The rupture of the eDLP envelope appears to be directed by conformational changes in the spike protein VP4, which transitions from a highly flexible, premature conformation in the eDLP to the partially dimeric mature conformation observed in the TLP. The disruption of the transient envelope and the assembly of the VP7 capsid, which locks the VP4 spikes in place, are proposed to be driven by this transition. The process by which the newly assembled TLP, within COPII-derived vesicles, moves outside the cell has not been thoroughly investigated (C7). However, in different systems, rotavirus has been shown to exit the cell by lysis, active secretion from the apical cell surface before cell lysis occurs, and in the form of extracellular vesicles containing viruses originating at the plasma membrane, which are responsible for the vesicle-mediated en bloc transmission.