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. 2023 Apr 25:1–17. Online ahead of print. doi: 10.1038/s41579-023-00889-z

Fig. 4. Multivesicular endosome-dependent biogenesis of quasi-enveloped virions.

Fig. 4

a, Hepatitis A virus (HAV) capsids assemble from 60 copies of three major capsid proteins in close association with the limiting membrane of endosomes into which they bud. An 8-kDa C-terminal extension of the VP1 protein, pX, recruits two endosomal complexes required for transport (ESCRT)-associated proteins — programmed cell death 6-interacting protein (ALIX) and tyrosine-protein phosphatase non-receptor type 23 (HD-PTP) — as well as the ubiquitin ligase ITCH. This induces assembly of ESCRT-III complexes containing multiple charged multivesicular body proteins (CHMPs) and vacuolar protein sorting-associated protein IST1 homologue (IST1) that pinch off the membrane, creating an intraluminal vesicle (ILV) that contains single or multiple HAV capsids93. Ubiquitin (Ubi) tags conjugated to viral or ESCRT-associated proteins by ITCH may facilitate this process. ALIX-interacting late domains also exist within the VP2 capsid protein (YPX3L motifs)91. Multivesicular endosomes (MVEs) containing multiple HAV-laden ILVs traffic to the apical (shown) and basolateral (not shown) membranes, where fusion leads to the release of quasi-enveloped HAV (eHAV) into the biliary tract and sinusoidal blood, respectively. High concentrations of bile acids strip the membrane from eHAV in the bile canaliculus, resulting in faecal shedding of naked virus (nHAV). b, The biogenesis of quasi-enveloped hepatitis E virus (eHEV) is similar to eHAV, with capsids assembling from 180 copies of ORF2 protein interacting with ORF3 protein associated with the cytosolic leaflet of endosomal membranes via its palmitoylated N terminus121. ORF3 protein recruits the ESCRT-I protein TSG101 to initiate ESCRT-dependent membrane scission and ILV formation104,122. Additional interactions of ORF2 or ORF3 proteins with other ESCRT-associated proteins seem likely to occur but have not been identified. eHEV ILVs generally contain only a single capsid, possibly reflecting a more integrated process of capsid assembly and quasi-envelopment. Fusion of the MVE and plasma membranes and release of HEV virions is similar to HAV.